Bev -
That sounds good. I took my 2 week trip last May and loved every second of it. I too took a car and explored after my initial 3 days in Edinburgh.
I will say that probably > 90% of the things I went to did not require advance reservations. Of the ones that did in my case most of them you would do the reservation the day of or the day before (mary king's close tour, Maeshowe for example). Others were typically easy to get in and get started with. Nothing like what I've heard for many places in Italy, but it's always worth checking - your guidebook may alert you to the places where you need to schedule ahead as well.
If you haven't checked it out already I'd suggest a visit to http://www.undiscoveredscotland.co.uk If it's in Scotland it's probably on that site somewhere. Between that site, TripAdvisor, checking in on this site and some guidebooks I was not left wanting for anything.
Happy travels, and I look forward to seeing what you come up with for an itinerary. If you post it here you'll be sure to get some good advice for how workable it may be. Just be aware it takes longer to drive almost anywhere than you think it will (or longer than Google and such say it will) - and that doesn't count slowing down to enjoy the scenery!
Eric