I have 25 CSC left over, what can I do with it?

I think it costs 25 CSC to autotrain your pet, though it does also cost SP and that price depends on how high their stats are.

Yeah it's 25 CSC, but that'd be a last resort because I'd like something more tangible

You could just hold onto it. If you ever get a Morty card whilst questing, doing 5 quests with one of those activated gets you another 25 CSC, and if you do all your quests depending on your luck, maybe you can get 1 or 2 per day (there's also a chance you get none, but on average I would say you're bound to get one) so you can build up more CSC to buy something you want rather than spending your loose change on something you might want less :)

I just sold off a Morty card D: Thanks for the info, I didn't know CSC was available from quests, I re-created my account 3-4 days ago and have yet to see any

25 CSC is worth around 2.5M, so you could look around trades to see if anyone wants to trade their items for CSC.