If you are planning on taking the direct Stockholm-Oslo train (only 2 or 3 per day), then you might consider Karlstad, which is one of the stops along the route and almost half-way. It's a pleasant university town on the shores of Lake Vänern. There's a few small museums, decent restaurant and bar scene and you can take lake cruises in the summer.
Another option would be to stop in Gothenburg for a few nights. It's the 2nd biggest city in Sweden (so not a small town), but it's not nearly as big as Stockholm and has lot's to see and do. Rick doesn't cover either of these cities in his books, but you might check out Lonely Planet Sweden for hotel, restaurant and tourist site recommendations.