I live very close to Balmoral Castle and can confirm only one room of the castle proper (the ballroom) is open to the public. There are exhibitions in the stables area, a short film to see and you also get the use of an audioguide to take around with you. This will add more to your experience and help you understand the history of Balmoral and how the modern estate is run. My advice is to spend a couple of hours at least on the site to get the most out of it.
There are four marked walks in the grounds and gardens which should not be dismissed, unless the weather is very wet. A combination of the blue and red walks will take you away from the castle gardens and into the woodlands where you have a good chance of seeing red squirrels and birdlife. The yellow and green walks are more on the flat and take in gardens and a nice walk near to the river Dee.
It would be a good idea to combine a visit to Balmoral with a distillery visit to either Glenlivet or Glenfiddich on the same day. Both to my knowledge have free entry and you do get to try a dram at the end of the tour. However Scotland has very strict drink drive laws now and whilst a dram at a distillery might not put you over the limit another drink somewhere else afterwards might.
The drive from Glenlivet along the A939 to Balmoral is fabulous and I recommend it to anyone.
Hope this helps.