The best part of the town is the walk around the completely preserved defensive wall

but, be warned! There are only two places to get on and off the wall! Ant the stroll is not short, but worth every drop of sweat (and it's bund to produce a lot of sweat- expect temperatures in the 30 celsius range.
Try and see if there is a tour to Prevlaka- it's border country, but has an extremely well preserved complex of austro-hungarian fortifications.
As Maki said, the surrounding countryside and the islands are also well worth a visit, especially Mljet!
A point of interest is Ston- it's on the isthmus (sp?) of the Peljesac peninsula and also boast very well preserved fortifications.
As for model shops, I know there aren't any specialised, but you might be lucky in one of the local toy shops.
have a great time- It's a beutiful city!!
Cheers,
Tvrtko