Dungeons and Dragons

I limit it to things the character would have realistically encountered before.

It could be fun to create/search for a d100 roll table for what they get, where there's a small chance to get something "broken".

Realistically, the real power from conjure animals comes from the "summon a horde of beasts" aspect of it. Without that, most choices are going to be pretty underwhelming. If you're looking to avoid the "swarm" aspect of it, I'd probably just go with single cr2 creatures, like giant crayfish, polar bear, or giant elk

Bears, wolves/direwolves, giant eagles, hell the druid in the game I run summoned an auroch and rode it into combat once. I limit it to beasts the player has encountered personally or would reasonably know enough about their anatomy to summon, and then just limit what I expose them to.

As far as preventing cheesing with 8x small beasts and turning the battlefield into chaos? I recommend enemies with AOE attacks to clear the field.

Here's an alternative take based on how I've played a druid in a multi-year campaign:

The druid can pick whatever animals they'd like, but:

1. Each animal type can only be chosen once.
2. The chosen animal should be thematically appropriate for the environment (e.g. don't choose elk in a desert or snakes in the arctic).

My DM let me choose whatever animals I wanted except for dinosaurs. I didn't want to abuse that by spamming 8 wolves every encounter so I came up with these self-imposed restrictions to try to make things interesting. I think that the "each animal can only be used once" limitation in particular was very interesting because it gave the spell longer term strategic implications as well -- "is this encounter scary enough that I want to burn one of my stronger summon options".

My flavor for the restriction was that, since the animals are actually fey spirits, they didn't like being "taken for granted" and always summoned in the same form. They would only respond in the way you ask if your need was truly great