I'm planning to use +1 consumable item bonus tokens that stack with pretty much anything else, but not themselves. They can be applied to any d20 roll.
I'm gonna use poker chips
If someone demonstrates behaviour I want to encourage, they get one of these.
The tokens don't stack with each other, but stack with pretty much everything else. You can trade three +1 tokens in for a single +2 token. And three +2 tokens for a single +3 token.
YMMV
Edit: [MENTION=19265]Connorsrpg[/MENTION] I like your ideas, consider some of them stolen.
This is roughly what we do, although they are fixed at +2 and you can't trade up.
Given for in and out of game cool stuff.