In reply to Neil R:
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Sigmaringen was a reasonable size and may have had a swimming pool.
Sigmaringen has an open air pool (with an 80m water slide) in a park on the bank of the Danube .
In reply to the OP
Canadian canoe paddling is a top activity on the Upper Danube (as on most Bavarian rivers of any size); Google gives lots of results for rentals. Higher up the Danube is quite small and is ideal for ordinary paddling (of the rolled up trousers sort!) and general messing about in a river.
I have paddled (canoe) quite a lot of the Upper Danube but have never climbed there (because I lived in the Southern Frankenjura!) There are extensive bird bans, but there is a lot of rock.