Imagine the following situation:
A friend asks you if he/she can borrow your bike for a day. You don’t mind it. How would you let your friend know it’s OK for him/her to take your bike?
Here’s a few options:
Sure, no problem at all.
Absolutely!
Yeah, go ahead.
It’s all yours.
Feel free.
Help yourself.
Take your pick!
P.S. Naturally, you can use these expressions in any other situation similar to the example given here.