I have only a few retail stores, often I end up with a mini mall instead of one shop, and I normally let the game run it, after I had a sim establish it.
My input is mainly that I would try to find something world specific. The only collectable I heard of is those small marbles, which probably won't sell for much unless they include a few very rare items. Being a tourist I would probably not see marbles as a typical Tomarang souvenir, though :)
You can sell Mold infected gnomes? Guess sims will buy just anything ;)
Or maybe just wall/table deco so that visitors can actually buy something to bring home. Taking and selling photos with local motifs might work too, combined with a typical gallery or souvenir shop.