Couple of recommends :
Callander and the Trossachs : auchenlaich farmhouse service second to none, on an NCR, folks that run it do dinners etc and kids can be fed earlier if required, they got DVDs washing facilities etc and are bike friendly.
Also, for something different there is an old railway sleeper carriage just north of Dornoch which you can stay in. For around £10 per night each, each old carriage (think of the old films where there is a sliding door into a small compartment with 3 seats either side of the window) has the three seats one side and bunks on the other. They have cooking facilities, BBQ, lovely gardens etc.
Aviemore in summer would be a must but will be very busy. Anywhere around crianlarich etc for west highland way. Would definitely recommend Oban, seil island,etc for lots to do.