“I'm up for anything.”
You're with your friend. You're deciding what to do tonight. He suggests going to a dance club where his friend is DJ'ing & asks if you're OK with that. You don't care what you do, and want to show that you're having fun. You say this enthusiastically.
I'm up for anything.
When you are "up for" an activity, it means that you are willing to do it.
When you ask a friend "Are you up for ___?", you want to know if they want to do it, and whether they have enough energy for it. For example, if someone asks:
You want to play a game of basketball?
...if you're too tired right now, you could say:
I don't think I'm up for it right now.
This means that you are OK with anything that someone suggests:
A: What do you want to do tonight?
B: I'm up for anything.
This phrase sounds fun, positive, and relaxed.