Yep, museum sounds good... or if she has a particular interest, you could visit some shops surrounding that interest (like my DD is into art, so art supply shops are her idea of heaven), or just some general shopping/browsing in shops she would like (antique shops can be good fun to look at, but it would depends on how baby friendly they are I guess, if you have a stroller for your DS). Otherwise, lunch at a favourite restaurant always goes down a treat... or just getting some takeaway and having a little picnic lunch in a park.
Have fun!