The Augustow Canal[0][1], on the UNESCO World Heritage tentative list, is a nice one that was hailed as a technical marvel in its day. It was originally constructed as the result of high Prussian customs on the Vistula following the partition of Poland. (The Vistula was the principal trade route for Polish trade for centuries. The wealth of historic ports on the Vistula river are a testament to this fact, with something like 60% of Polish grain exports, the most lucrative export by total revenue, taking this route. The partitions divided the river and its ports between Russia, Prussia, and Austria, hence why the canal was built in the Russian partition).

Today, it is a nice scenic waterway for kayaking and other vessels.

[0] https://www.augustow.pl/en/augustow-canal

[1] https://www.tripadvisor.com/Attraction_Review-g274739-d69939...