Stadtmuseum Schloss Lübben gives the Spreewald a useful indoor anchor. Set in Lübben's castle, it covers local history, landscape, town life and the Sorbian/Wendish culture that gives the region more depth than postcard canals and pickles.
It is a sensible stop before or after a boat trip, especially if the weather turns. The museum helps explain why the Spreewald's waterways, villages and bilingual traditions fit together, rather than leaving them as pretty but disconnected details.