We Interrailed through Germany last summer- it's a great country to do it in.
Are you sure you need a hotel with a pool, because Germany seems to be the world capital of outdoor public swimming pools (Although I hear Austria is pretty good too!) They're called "Freibaden" or "Sommerbaden", and they all seem to be huge, cheap and have massive grassy areas for chilling out in.
In addition, there are swimming lakes in some places. I reckon Berlin has to be the capital of this- it's ringed by hundreds of great swimming lakes! (We swam in the Schlachtensee). And I would definitely recommend visiting Berlin.
Our second favourite was Nuremberg (visited a massive outdoor pool that just happened to be next to the old Nazi Party rally grounds. Which was an interesting juxtaposition). The castle and medieval centre are lovely.
We also stopped in Cologne for the night, which we liked but felt we'd seen all the sights in one morning (entirely possible there were a load we didn't discover though).
Wasn't so keen on Heidelberg- it seemed a bit over touristed and over manicured.
If I'd had more time I also liked the sound of Lubeck, Trier, Dresden, Regensberg, Aachen and Ulm.