At my son's last party, we bought some colorful freezer mugs from Party City and used them to serve the drinks during the party and sent them home as favors later. The kids loved them.
This won't work for you, but anyone doing a summer pool party, we bought noodles for each kid and that was their favor.
As you can see, I prefer to find something that they can use at the party and then keep it as their favor.
We've also done scavenger hunts where they find their favor: things like those small packages of legos, small game or puzzle, nail polish and emory boards for girls....