Any bets?

ToBeThatGirlIDreamOf

My best friend and I want to do a health or fitness bet for about a month.
Although, we have no idea what would be a fair bet.
She plays volleyball, I run 5ks, she runs but not as far, and we both strength train.
Other than that, we're pretty much equivalent.
Any ideas on what we could do?
Or a reward for the winner?

ToBeThatGirlIDreamOf

Or maybe a health challenge would be more appropriate. But ideas would still be appreciated.