Tough one. Assuming I couldn't take any instruments, and that I couldn't take it anywhere to be examined, so I wouldn't have evidence and I couldn't do much research, I think I'd go to Mars. I've always wanted to go there, and while there are many things I'd like to see if I could go to multiple locations, if it was just one, I think I might go there. I think I'd write "
Kilroy was here" on one of the rovers and grab some rocks. (That assumes they have a spacesuit that fits, of course.)