...someplace large where there would be an awful lot to do. The obvious answers are Target or someplace such as a giant sporting goods store. Both viable options, both not particularly thrilling though. What store really would be thrilling? There would be some neat food establishments that might warrant an overnight visit but that would get old real quick. A movie theatre probably isn't considered a store yet it would be fun to be in overnight. A store similar to FAO Schwartz would be fun although they really do rock a lot of really young child toys so it might get old playing with stuffed animals after a couple hours.
Basically the decision would have to be made based upon what would be the most fun for the longest amount of time. In an ideal scenario it would be a Mall where you could go into all sorts of different stores but since that clearly isn't what the question is asking I'm going to go back to the first, obvious answer; Target. It's large and has all sorts of junk in it that I could play with (bikes, mini-motorized cars, Big Wheels, etc.). There are also video games and giant televisions. They have food processing equipment and food, as well as microwaves and mini-ovens (plus if the fire alarm is turned off I could just grill in one section of the store). I could shower and groom with all sorts of fancy pants products I would never buy (again, assuming everything would be mine for the taking). I would stay busy and be able to move around a good amount, sounds like the best choice I could make.
Too bad this sort of thing only happens in bad movies from the 80's.