I see that the seaside town of Ostend is a very short train ride from Bruges. In your opinion, would this be a good use of a free day in Bruges, or would more Bruges back-dooring and a bicycle ride to Damme be a better use of that one day? Thank you in advance for your wisdom! We are looking forward to our Heart of Belgium tour four weeks from now!
If WW2 and a bit of WW1 too is of interest you can visit Atlantikwall Raversyde, a stretch with well preserved gunposts and trenches along the coast. For this you can take the coastal tram from the railway station in Ostend to Middelkerke and stops nearby the domain. You can visit the Amadine, a former fishing vessel and the three mast sailing ship Mercator. The home of James Ensor in case painters are of interest, well known for it’s expressionist and especially surrealistic paintings. Further Ostend has a serious number of outdoor restaurants, so close to the sea the best place for seafood, however no experience with any of them.
You can combine Ostend with De Haan, a short ride with the coastal tram. De Haan has beautiful Belle Epoque architecture and still old fashioned villa’s. It was also home to Albert Einstein for a half year before he went to Princeton. The TI is inside the charming tram station and De Haan is also good for outdoor dining.
Actually Ostend is for the rest far from charming to my opinion. A whole lot of ugly apartment blocks, but you can experience the Belgians in their own habitat, what can be worth doing. De Haan is way more attractive.
concerning Damme you can bike further along the canal to Sluis just across the Dutch border and include a detour of a few kilometers to tiny but charming Sint Anna ter Muiden. Closer to Damme a detour to Oostkerke is also worth doing.
So this is what both options have as far as I know to offer, what appeals most is up to you.
Your OP suggests that outdoor activities are more important to you than, say, art or history. Many people would prioritize Ghent or Antwerp (I mean from Bruges) ahead of Ostend. While Ostend has an exceptionally wide, sand beach, I don't go to Europe for beach time, myself. Except for a few historic structures (like the Ensor Family church, and a synagogue that looked very locked up, it's a mainly reinforced-concrete, postwar town.
However, the coastal tram (which I only rode as far as Our Lady of The Dunes) may be a nice opportunity for hiking, biking (check tram rules for bikes), or WW II historians. I get the idea that Ostend is a bit of a beer party on summer nights, but I also saw an expensive yacht basin near the train station. But I don't know that first-hand.
I enjoy Ostend, but in my opinion it doesn't require a full day. I like to go to the fish market and hit the prepared fish stands, which are just outside the train station to the right. The Mercator ship is interesting and also quite close to the train station (look left exiting the train station). Then use the Kust Tram just in front of the station to visit another town on their route. De Haan is very pretty!