Everyone is different and has varying degrees of comfort when it comes to trying out new sexual things. I get bored easily when it comes to sex, so I'm always trying to find different things to try. One thing that can be fun (though I'm in no way advocating because you can get into a lot of trouble for it
) is sex in public. I can't even count the amount of times me and whoever I was dating went at it during the middle of the day in a park, or a deserted hallway, or a parking lot, or where ever we could find. Hell, we had sex in the middle of a park field during the day. In retrospect we could have gotten into a LOT of trouble if a cop had found us (indecent exposure in an area where kids play.... though there were no kids in sight lol), but it adds to that extra rush you get.
One thing you can do is simply visit a decent sex store and browse the different items they have for sale. There will probably be something that catches one of your eyes that might be worth trying out. Try things like bondage tape/handcuffs or role playing. You can get as creative and kinky as you want. It's not everyone's cup of tea, but if you get the right situation going, it can certainly add something.
As for having a hard-to-get-off girl, I've been there. One girl I dated for a few months was
the hardest person I've ever been with to get off. She was super sensitive when it came to anything touching her, yet if you did anything more than a gentle caress, it would hurt like a son of a bitch. The only thing I can suggest for that is to persevere and don't give up too early. Plenty of things will probably get her motor running but it's that extra little bit that pushes her over the edge that is the hardest trick to figure out.