Just returned from Scotland and must disagree with the Rick Steves guidebook assessment that Inveraray Castle is impressive from a distance but "dull" inside.
We found, first of all, that the gardens were spectacular -- I couldn't stop taking photos of the lovely colors and blooms. Then when we went inside we found all kinds of fascinating and beautiful artwork, furnishings, arms & armor, and architectural details. It is a modern castle, so if you are looking for medieval underpinnings you'll be disappointed, but if you have any interest in 19th- and 20th-century history you can't go wrong spending an afternoon there.
Our purpose in visiting Inveraray was to see Kilmartin Glen stone circles, which were great once we got the map for 1 pound at the museum by the church in the village of Kilmartin. In Inveraray we also were extremely happy with our accommodations and breakfasts at Brambles, and dinners at the George Hotel.