pocket it and hang around to see if anyone came back looking for it. I'd wait about an hour. if nobody came, I'd spend it. if somebody did come looking, I'd like to say I'd give it back, but that partially depends on their demeanor. I mean, who carries $1000 cash? if it was some shady character they'd be SOL but I'm not sure how I'd determine that... not by looks alone. I would never take it to the cops, though.
Hide it well :)
Start a savings account that no one else would know about.
Cry - I lost a wad of $2,000
spend it :)
Put it in the bank :/