The Gurkenmuseum in Lübbenau is exactly what it sounds like, and it takes itself with exactly the right amount of seriousness. The exhibits trace the history of the Spreewald pickle from its origins as a subsistence crop in the 17th century to its status today as a protected regional product sold across Germany. You will learn more about cucumbers than you expected to want to know, and you will almost certainly leave with a jar.
The museum is small but well put-together, with sections on the ecology of the Spreewald soil (why the cucumbers grown here taste different), the traditional pickling methods, and the industry that grew up around them in the 20th century. It is housed in the Spreewälder Gurken factory complex. Admission is cheap; the gift shop is the real reason to visit.