I suppose that depends on what you've already tried. Sex toys can be a fun addition to lesbian sex, and there's a huge variety out there. If you've never used a dildo, you should consider it - sometimes one woman can't reach her girlfriend's G-spot with her fingers alone, and using a dildo can hit it just right. Vibrators are also fun, so long as you and/or your girlfriend aren't too ticklish.
If you're wiling to try it, bondage can be a great addition to "usual" bedroom fun. Depending on who is the more submissive partner, she can be tied up with handcuffs (fuzzy or not), a necktie, a scarf, or really anything that can hold her hands above her head. Blindfolds are fun too.
Another fun thing to try is female ejaculation; it's usually something that women have to actively attempt to do, but can be very rewarding when done right. I've heard that it's advisable to put down a towel before trying it though, as it can apparently get pretty messy.
The last thing I'd suggest is not being afraid to try foodplay - things like honey, etc. can easily be licked off a willing partner and it adds a bit of spice to things.
Hope this helps.