I will be going to Bruges and Holland in September and would like to visit exciting, fun, off-the-beaten-path, or unusual places or experiences (including restaurants, coffeeshops, stores, museums, towns, hotels, etc.) I would appreciate any advice or recommendations anyone can offer.
Thank you!!!

In Belgium - the town of Ghent is worth going to and exploring.

In the Netherlands - Delft, Den Haag, Leiden, and Rotterdam.

In Holland: choose a dry day. No matter where you are, rent a bike at the train station and bike out of town. You can get bike route suggestions at any tourist office (VVV). If you have time, spend a night or two somewhere else. Great bike trip: along Kinderdijk near Rotterdam (touristy, but most of them are in tour buses!); through the dunes between Haarlem and the North Sea (water warm enough for swimming); north from The Hague along the coast, Delft to Hoek van Holland past endless greenhouses, then back to Delft via the tiny town of 't Zwet for a fabulous dinner at the half-submerged restaurant Zwetheul; the National Park Hoge Veluwe (near Arnhem / Otterloo) and the Kroeller-Mueller art museum. Or head for Maastricht and explore that Dutch/Belgian corner.
Go to The Hague to see the Queen ride in her Golden Coach to address Parliament on the third Tuesday in September!

If you have a car, go up to Middleburg, definitely off the beaten path. In Brugge, we really liked the Time Out cafe, which is a bit away from the central action. There were a few jazz/nightclub venues along Langstraat that were more locals oriented.

Another rec: Heer Halewijn wine bar on Walplein. It's fairly new, so not as well known as some of the other cafes in town, but still centrally located. We liked it a lot and it's not that touristy.

The museum of mechanical musical instruments in Utrecht - all sorts of weird and wonderful machinery that (in the end) they let you dance to.

I second btilke's suggestion of Middelburg. While you're there, don't miss the nearby old port town of Veere.