I'm about to hit 5th level druid giving me access to Conjure Animals. There are currently like 598 different animals for the DM to choose from (collectively) for Conjure Animals. Would it be feasible to suggest to the DM to make a list of 20 animals from each challenge rating (0-2) then roll a D20 to see which animals are summoned?
My thought is it's a reasonable amount of animals to have on deck, and random enough to still be fun to use.
DMs will usually allow players to choose the animals summoned just for the sake of reducing their own work. If your DM doesn't let you choose, he can figure it out on the fly the first time and make his own list after.
When I played a druid, I used to prep 3 new bags of miniatures each session so as to never use the same animal type twice (I have a *lot* of miniatures). I also put printed stat cards in each bag, so as not to slow down the game.
But for pure convenience: nothing beats a bear. It's generally the best animal at every level, and only requires one miniature.
Conjure animals as tweet edited still allows a druid to pick the challenge rating of the creatures summoned. (that part was in the spell description)
You can summon creatures from different environments, no restrictions are on the spell for that, but it makes thematic sense for them to come from the type of environment you are in. If this theme is agreed it makes thematic sense for you to damn well pick your creatures, and have them statted and ready to run in advance so you dont slow anything down.
If what you summon is random from within the CR provided you can minimise the terrible by again ensuring that creatures summoned are approrpiate to the environment. Target the air and you will only summon flying creatures, the walls, climbing creatures, underwater, aquatic or amphibian creatures.
Props and obstructions can guarantee the more unusual creatures: Trying to summon two cr1 creatures inside barrels will really limit the list, usually ending with a creature that could only fit because of an unconventional skeletal structure due to the size of the creatures. You end up with Rocktopus (out of the abyss) air breathing octopi that can climb, restrain, camouflage (and fit in a barrel so oddly higher AC, comes in useful)