Yeah... if OP can delay till August, you've got the
Fringe Festival (which is fantastic), and the
Military Tattoo (a spectacle of bagpipes). You can book tickets for specific Fringe shows online (it's like 3000 events in 300 venues over 30 days) and there will be street buskers trying to get you to come to their shows, often in some entertaining fashion. I cannot recommend the Fringe enough. I went in '96 and it was both my first solo vacation and my favorite. There's also a film festival and an international music festival in August.
I don't know how mobile OP is, but the hike up Ben Nevis (the highest peak in UK, but basically a big hill in New Hampshire) was nice. I met some curious mountain sheep on the trails.
If you're renting a car, keep in mind many of the secondary roads in the north are basically one lanes with cutouts every so often so car going one way can get out of way of car going the other direction.
Grampians are gorgeous. Kinda like Green Mountains in Vermont, but with far less civilization. Also recommend just laying outside at night and stargazing as there's not much light pollution outside the big cities. There's not much to do in larger towns like Inverness or Ft. Edward, so your nights will be free ;-)
Keep in mind once you're outside the big cities, restaurants close early, so plan to eat around 6pm. Try the haggis (seriously). Fish & chips and curries are typically good anywhere, but other food is pretty meh. Other than a Turkish restaurant in St. Andrews, I pretty much only ate pub food, so no recos on restaurants.
If you can't do August and have to do July, there is a
Jazz & Blues festival from July 11-20. But if at all possible, go in August - and book your accommodations early as the place sells out.
