“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.
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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.