Similar to lots of other suggestions. We went to Zell am See in the Austrian Alps this summer with our 9 year old son.
Flew Manchester to Salzburg and got a taxi from there to Zell but there is also a train.
Stayed in a hotel which was lovely-beautiful hotel, immaculately clean and had a pool and spa area. Was quite a small hotel which was lovely as it didn't feel impersonal or over busy. Food was amazing!
So much to do in the area - castle, visit Salzburg, indoor and outdoor pools in Zell, gorges you can walk up, mini golf, boating on the lake, high mountain reservoirs plus cable cars up various mountains where you can walk.
We used buses and trains to get around and they were great.
One thing I would say is, Austria can be expensive. Our hotel gave us a summer card which included free entry to lots of attractions and the cable cars. Plus free travel on local buses and trains. Saved us an absolute fortune so well worth looking for somewhere that offers this. We were also half board in the hotel so all food was included.
Loved it so much we are going again next year.