Good Morning, America (How are you?),
I didn't get to see half of what I wanted to while I was in Scotland, so consider some of my recommendations to be something of a pathetic effort on my part to live vicariously through you. I'll tell you about all of the places I wanted to go to but didn't.
Isle of Skye: Absolutely breathtaking scenery. It sounds like a fantastic place to get away from the commotion of Edinburgh and Glasgow. Apparently there's still quite a bit of Gaelic heritage here as well.
Ullapool - This place sounds as wonderful as the Isle of Skye. I found this
site to be quite informative - I would recommend perusing it on your own to find out what interests you.
Loch Lomond - Once again, breathtaking scenery. There's a reason that there's a famous folk song written about the place. It's the second-largest loch in Scotland and has several islands which are accessible by boat. They even have mini cruises that will show you around the loch in an hour or so.
Now, the following places aren't exactly in the highlands, but they're places that I've been to and can thus recommend more knowledgeably.
Edinburgh - There is a lot to do here. You can check out Edinburgh or Craigmillar Castles, stroll along the Royal Mile, visit old cathedrals on the Royal Mile, visit the Elephant House Café (where J.K. Rowling began work on Harry Potter and the Philosopheŕ’s Stone), visit various art galleries, and hike Arthur's Seat (highly recommended). I would personally recommend Castle Rock Hostel - it's right by Edinburgh Castle on the Royal Mile. It's well-kept, has gender-segregated rooms (this isn't exactly enforced, though), and coordinates day trips to sights up in the highlands.
St. Andrews - This was the main destination on the Castle Rock Hostel tour that I ended up going on. There's an old cemetery among old abbey ruins that was quite interesting. St. Andrews also boasts having the oldest golf course in the world. It is also home to Scotland's first university, the University of St. Andrews, founded in 1413. This is also where Prince William studied a few years ago. Throw in a sandy beach and superb ice cream stores and you've got a fantastic day trip.
Glasgow - If you do some research and decide what you want to see before you go, I'm sure that Glasgow could be a rewarding destination. It has several theaters, museums, and art galleries to choose from. Unfortunately, I wasn't able to experience any of this as my friend and I didn't plan ahead, couldn't decide what to do, and got lost.
As for your hostel question, I would recommend booking hostels in advance, particularly if they're in smaller cities. They can get pretty busy at the height of tourist season. That said, though, you could probably get along okay with being a little spontaneous; most hostels will still have some open spots just a couple of days in advance. You just want to make sure that you have another hostel lined up before leaving the one you're in.