So I've DMed for a couple years, but my players in that game were more into a decidedly railroaded type of game. They liked the story I'd written and wanted to experience it without muddling it up. I still gave them choices throughout, but they were definitely more along the Choose Your Own Adventure line rather than anything truly open ended. 

I'm in the midst of planning a new game with a different group of friends, and I'm planning on this one being a little more player-driven, open-world type of adventure. There's no one "main plot" but there are going to be various villains that the party might choose to face off against eventually. The main thing I'm trying to do this time around is to do a lot early on to populate the world with challenges, adventures, dungeons, and things to come across so they'll have real choice on what to do, rather than artificially limiting choice through unpreparedness.

While my last game was almost entirely homebrew, I'm relying more on resources like DMsguild to find content to put in my world (since pre-populating enough adventure with all homebrew is ambitions to say the least). Anyone have any good suggestions either for running a sandbox adventure, or suggestions for content I can look up?

I've already got and am planning to use Tales from the Yawning Portal for the bigger dungeons, but for suggestions I'm not looking for anything bigger than what's there, so not any of the mainstream WotC published adventures, and I'm not looking to spend a ton but would be willing to drop 1-3 dollars on a few fun dmsguild pdfs if I can't find good free content. The campaign setting is nebulous, but I'm going for a sort of back-to-basics style medieval fantasy adventure, since I'm newer running a sandbox game and some of my players are kind of on the intermediate side of being beginners. 

Thanks for reading and thanks in advance for your suggestions! 

how much time do you have to plan before session 1

no matter what, start by making a map with 3+ large cities, then add a few points of high interest

We're gunna start sometime after the new year, so I've got at least till then. 

oh, you got plenty of time then, if you were on a time crunch i'd say only plan areas near the starting town, but since you've got at least a month i'd say begin around the starting town, up to about where they could get in 2-3 sessions, then plan future important areas, if you have been planning non-stop (that's unhealthy, don't) and somehow still have like 15 days left, then you could even get a few more areas in, make sure when you make an area give it some quests the party could get, heck i even design major map points around quests, then give that map point its own quests they could get inside

with your amount of time, if you have a healthy social and sleep life, id say you could plan 5-6 towns/cities and 2-5 quests in each

I posted this in another thread that has some food for thought for sandbox style games. Vedexant's post is also really useful, and a lot more thorough, to give you some further food for structuring an unstructured game.

I've been running a sandbox style game in the Oerth/Greyhawk, as a basis to get them started I ran the first two adventures from Ghost's of Saltmarsh, then let them direct themselves from there, exploration is mostly hex-crawl, which actually has worked better than I though; currently they've managed to get sidetracked in the Elemental Plane of Water and have had some super fun sea-voyaging play. The main thing's I've found useful are as follows:

Random tables are your friend. have a good mix (random encounters, npc traits, adventure hooks) and don't be afraid to add to them or completely replace them when they are stale; also make sure to add some entries that are relevant to the region they are used in. Ghost's of Saltmarsh has a good series of tables which generate islands, letting you decide if they are hostile, who lives there, and also has a table of plot hooks too. though remember Don't be afraid to just decide what happens if you get a good idea. tables are just inspiration, after all. WebDM has a good videio on tables

Have a few adventures in the back pocket.Tales of the Yawning Portal and a few other dmsguild or assorted adventures should do you fine for a good while, pepper in some shorter ones or ones generated from random table rolls and you should be fine. as a sidenote, you might have multiple adventures which cover the same level range, I've found that sprinkling some low level adventures into games useful and makes the players feel capable, but do it sparingly, overconfidence is a slow and insidious killer.

Give player's appropriate warnings. Make sure to let them know if they are going into a location which might be above their ability to handle, but if they get into a situation, don't make it impossible for them to get out.

Sidenote(opinion piece): 

Encumberance is enFUNberance. It really depends on the group, but I've found that encumberance has really led to a change in mindset of my players when it comes to both exploration and dungeon delving. it encourages players to use items or spells they might otherwise not use, however it's only really a viable thing to begin tracking at the start of a campaign, since keeping track of weights can be a bit nightmarish if you swapped over midway.

Hex-crawls. This depends on a few things, but if you have a map consider doing a hex-crawl. it allows you to add specific adventure locations as you go along and really emphasises the journey element of adventuring; only go for it if you think your players will like it though, as it isn't for everyone. if you do decide to sprinkle in some group checks and have some area descriptions ready just to keep things interesting.

Hope that helps, I got a bit rambly there so if you need anything clarified just ask.

Quote from TriWalt >>

I've been running a sandbox style game in the Oerth/Greyhawk, as a basis to get them started I ran the first two adventures from Ghost's of Saltmarsh, then let them direct themselves from there, exploration is mostly hex-crawl, which actually has worked better than I though; currently they've managed to get sidetracked in the Elemental Plane of Water and have had some super fun sea-voyaging play. The main thing's I've found useful are as follows:

Random tables are your friend. have a good mix (random encounters, npc traits, adventure hooks) and don't be afraid to add to them or completely replace them when they are stale; also make sure to add some entries that are relevant to the region they are used in. Ghost's of Saltmarsh has a good series of tables which generate islands, letting you decide if they are hostile, who lives there, and also has a table of plot hooks too. though remember Don't be afraid to just decide what happens if you get a good idea. tables are just inspiration, after all. WebDM has a good videio on tables

Have a few adventures in the back pocket.Tales of the Yawning Portal and a few other dmsguild or assorted adventures should do you fine for a good while, pepper in some shorter ones or ones generated from random table rolls and you should be fine. as a sidenote, you might have multiple adventures which cover the same level range, I've found that sprinkling some low level adventures into games useful and makes the players feel capable, but do it sparingly, overconfidence is a slow and insidious killer.

Give player's appropriate warnings. Make sure to let them know if they are going into a location which might be above their ability to handle, but if they get into a situation, don't make it impossible for them to get out.

Sidenote(opinion piece): 

Encumberance is enFUNberance. It really depends on the group, but I've found that encumberance has really led to a change in mindset of my players when it comes to both exploration and dungeon delving. it encourages players to use items or spells they might otherwise not use, however it's only really a viable thing to begin tracking at the start of a campaign, since keeping track of weights can be a bit nightmarish if you swapped over midway.

Hex-crawls. This depends on a few things, but if you have a map consider doing a hex-crawl. it allows you to add specific adventure locations as you go along and really emphasises the journey element of adventuring; only go for it if you think your players will like it though, as it isn't for everyone. if you do decide to sprinkle in some group checks and have some area descriptions ready just to keep things interesting.

 

Hope that helps, I got a bit rambly there so if you need anything clarified just ask.

 

 this is very good advice, encumbrance  is hit or miss though, ask your players about that one, otherwise this is really good

Thanks! A lot of good stuff here. I looked up how to run Hex crawls and found this article too, which is really useful: https://www.runagame.net/2014/03/the-hex-crawl.html?m=1