Well, for about a week I'd recomend you start in Edinburgh. The castle is definitely worth a visit. You get an idea on the history of Scotland and some nifty stuff like the crown jewels. There's also plenty of pubs and museums and guided tours including their famous "Ghost Walks."
Because of your time frame, I'd reccomend Fort William, which is a pleasant town smack in the middle of the highlands. It's big enough to have plenty of accomodation and small enough you get the small town feel. It's on the edges of a lake (Loch Linhe) and there's gorgeous scenery around, including majestic Ben Nevis (Highest mountain in the UK). It also about 20 min. from gorgeous Glencoe. Near Ft. William you can also visit Glen Nevis, which is a lovely glaciar valley. The area is so beautiful it has been used for numerous films like Harry Potter (all of them), Highlander, Rob Roy, etc...
This is what I would do... you get lakes, castles, heaps of activities both in Edinburgh and the highlands. And the scenery really is breathtaking.
I wouldn't reccomend Loch Ness. It's just a big lake surrounded by Nessie tourist shops. Also, on the bus ride from Edinburgh to Ft. William you get to see the "bonnie banks" of Loch Lommond.
About accomodation, well, I guess it depends if you have your own transportation. If not, accomodation is plentiful, and for your time frame, it might be easier to stay in a hotel. Check out the Visit Scotland website, they're pretty helpful with that and can offer some ideas.
Hope this helps, best of luck!