So a few sessions ago I had ice storm prepared and we ran into some undead that were immune to cold damage. I swapped it out for something else as I think we will have more undead going forward. 

what would you suggest for combating undead with a Druid? Not sure what we will run into but want to make the best of it. 

Depending on the type and quantity of undead you're facing, Spike Growth is always nice.
I'm a fan of using Conjure Woodland Beings to have a team of 8 blink dogs with you, though there are a few other low CR options that are definitely worth sprinkling in.
Obviously, Wall of Fire.

Though, depending on your party composition, it may be more practical to just play battlefield control and let your allies deal the brunt of the damage.

Anything that deals AOE stuff like Sleet Storm and Spike growth, as Memnosyne had said. When one of my druids was fighting undead (at high levels) I used the spell ice storm and my fellow druid used earthquake, so all the flying zombos were caught in the ice and all the ground ones fell into the fissures. The rest of my party at the time tried to make grenades which ended up costing the warforged his arm- But that's a story for another time.

Really anything that deals AOE or hinders the zombos.

At low levels, I kept popping Create Bonfires on them.

Know this is an old topic ThriKeen, but since it was revived, I would say Moon Beam is always a solid go to. Its one of those spells I always have in my prepared list, solely because so few creatures have resistant or immunity to Radiant, and if I am fighting a creature that is resistant or immune as a Good aligned PC, then I'm likely doing something wrong with my character RPing. Moon Beam also scales very nicely.. just note that it is a Constitution Saving Throw.

One of the biggest Advantages of Druids is that they can often have multiple Elements at hand and ready since they have a nice mix of them spread out over their levels. So if you run into things like undead immune to cold, I'd have to look if that's by the book or something your DM carried over, Then you can always easily hit them with lightning or fire or acid or some other kind of damage.

Moon Beam being a fine example of doing such.

Thanks for the replies in this old thread. I finally prepared moon beam but haven’t had a chance to use it yet. I previously hadn’t prepared it as I wasn’t sure how useful it was (action to move it) and if I wanted another concentration spell but quite a few people seem to like it. 

Moonbeam is good.  If your DM is following the monster manual, Zombies probably wouldn't even move out of it.  

Quote from ThriKreenWarrior >>

Thanks for the replies in this old thread. I finally prepared moon beam but haven’t had a chance to use it yet. I previously hadn’t prepared it as I wasn’t sure how useful it was (action to move it) and if I wanted another concentration spell but quite a few people seem to like it. 

 Despite it being an action to move and a concentration spell, it's actually quite powerful at low levels, as most creatures don't have a counterspell or dispel magic equipped. We were level 4 and I chose Moonbeam on one of my druids and using it, I killed a nerfed Rakshasa. Just a tip with moonbeam, try your best to use it in a way that makes it difficult to get out of. Even if it's a bunch of zombies, they might move out of it or disperse to attack targets- Now we got a problem. Try to use it on bunched up opponents (which is obvious), but also in tight points. In the same campaign with my moonbeam druid, we fought a camp of orcs who were stationed in a cave, so the rogue kidnapped the chief's favored wife, brought her outside, and I moon beamed the tunnel. That combined with a few other effects of what my friends were doing, we killed almost 2 dozen orcs in a few minutes, then pushed the rest of them.

LONG STORY SHORT be smart with your moonbeam, it can really change the tide of battle if done well, which is why I love druid so much.

Yup.  It helps if you have a beefy front line.  If you have 2 martial classes side by side, and you put the beam right in front of them with an enemy in the beam, the enemy can't move out of the beam without getting an opportunity attack from at least 1 of the players.  And of course, they can grapple enemies and pull them into the beam.

I discovered the disadvantage of the Moonbeam, which is against a Legendary Enemy. Which can immediately run out of it using its Legendary Actions.
The Legendary Action takes place out of its turn, so "When a creature enters the spell’s area for the first time on a turn or starts its turn there (...)" does not go up here.
Shame that I cast it at 5th level.

Yeah, I’ve used it a few times since my last post and I make sure either the target will at least start their turn or are blocked in or can’t move so I can get a few rounds on them. Not having initial damage is kind of lame, I used it one time and my party killed the thing before it could go off, but I know better now when to use it and when not to.