I think what will turn her on is being a man. Don't ask for it, take it. You know how to turn her on already, so go and do that. Persistence and attraction.

Also, if she's bored, I'd suggest reading up on some new tricks, or asking her directly what turns her on the most.

Women want sex just as bad as men do. The difference is that females are turned in a bit different way than men are. She's your girlfriend, you know best about what turns her on.