Temperatures can be difficult to guess, we stayed this year in Frankonia (Wuerzburg) and had around high twenties/low thirties but you can also easily have low twenties and rain. It is not an area with guaranteed sunshine but normally a bit better than the UK.
Holiday flats are normally easy to find, s/c is very popular but therefore good ones are gone fast. We do s/c a lot in Germany (we are Germans, ideal to get DD to practice) and this year it was the first time we had a complex with a heated pool, sauna and a hot tub in a small lodge. A pool is normally not that usual as you have a huge network of open air swimming leisure pools everywhere.
If you like mountains and hiking southern Bavaria (Garmisch) may be a good starting point, larger town with lots of things to do but equally easy to go everywhere and there is normally a lot to do for teens.
Alternatively, the lake, Chiemsee for example. I have never been there but it is suppose to be a lovely area.
Bayerischer Wald is a rural national park, lots of hikes, small towns, but gorgeous countryside.
North you have Franconia with Nurnberg/Wuerzburg and a more medium-high mountain range, so more walks than hikes. Around a 2-3 hour drive from Munich (depending on the day of the week).
August is high Summer holiday season in Bavaria and lots of other areas in Germany and a popular region, so there will be lots to do/see but equally lots of competition for s/c accommodation.
Another area you may want to check is Lake Constanz (Bodensee). They normally say it is the most weatherproof area in Germany, also higher temperatures than the rest. Saying that, when we stayed there we had 15 degrees mid August for some days. But I have been told it is a fluke.
It may be further to drive from Munich, we flew to Stuttgart but also a nice area with things to do.