Nipples, ear lobes, cock. However I really enjoy trying every place on the other guy until I find the places on him that drive him crazy. I really like when you find the place where it causes him to arch his back.
A lot of times, I have found I just started on top and slowly got used to it. I did little things so that he did not try to just force himself into me and let me work at my own speed. Then when you have gotten somewhat accustomed to length/thickness you can always go to other positions and let him start doing all the work. Makes the top even appreciate it all the more.