Shadowdancer is hard to pass up at least 2 levels in even if you go mostly straight. You get Hide in Plain Sight, permanent darkvision, and an extra +4 on improved Uncanny dodge levels, D8 HD for 2 levels, and you don't loose many skill points of class skills from Rogue (6 SP, fairly good list of skills)
Streetfighter in CA is a fairly interesting 5 level PrC that's pretty good at combat. You get initiative bonuses, decent skills, full BAB, a sneak attack die, and you can blow off half damage from an attack as non-lethal damage. Eventualy gets stacking uncanny dodge, and 4 SP per level.
I also listed a few others, but those all work well. Realy depends on your total concept. There is even the Thief Acrobat if you want to be more bouncy.