Bad Reichenhall is just across the Austrian border and close to Salzburg.  It has lots of old buildings and really nice pedestrian zone with shops and ice cream parlours.  There is also a great swimming lake called Thumsee on the road towards Inzell.

Chiemsee is Bavaria's largest freshwater lake and a centre for all sorts of watersports.  It's a good place to visit just to watch the sailing and wind/kite surfing but you can also take a boat trip across the lake to other towns on the shores including a visit to the castle.

This is a very beautiful lake with a pretty village on it's shore.  The best way to get a really good view of the lake is to take a boat trip or a rowing boat.  However, rather sadly, Königssee village has been spoilt by heavy commercialisation and is now somewhat of a 'tourist trap'.  It's still worth a visit to see the lake though.

To explore Munich fully would take a couple of days.  However, you can easily cover the city centre in a day visit.  The Hofbräuhaus is a must.  

Drive time from Flachau is about 2 hours.

These 2 large lakes are about 1.5 hours from Flachau.  The lakes are great for swimming, rowing and guided boat trips and the towns around the shore are interesting to look around.