Depends on what your parents like. I am "parent" age and am very fond of Asheville, North Carolina, Charleston SC, New Orleans, --and, well Rome The key to all these places is that they are strollable cities with lots of street life, interesting small shops, some good museums or historical sites, and comfortable hotels or B and Bs.
Things you should think about are
Your parent's energy level--some folks like to go all day and some cannot.
What your parents love--Are they shoppers, sunbathers?
What they like to eat--Gourmet or homefood?

It's a very kind idea and I am sure they will love whatever you choose because you gave it to them.