Scotland is a very cool place, my dad and I went there a little over a year ago to do a distillery tour and check out ruins. We went north along the east coast and then west to Dufftown and that was our home base for 4 days. We flew into Edinburgh and on our first full day there we hired a tour guide to take us on an abbey tour which are to the south of Edinburgh. They are all in different states of ruins, we went to Dryburgh, Kelso, Jedburgh, Melrose (this is where Robert The Bruce's heart was supposedly buried). Edinburgh Castle is very cool but at the same time very crowded. St. Andrews Castle is pretty nice but not a lot of ruins left and further up the road (short walking distance) is St. Andrews Cathedral and cemetery. Slains Castle was also pretty cool as it was right on a cliff but unfortunately this one is not secured that there was quite a bit of graffiti and liter around the area. Elgin Cathedral was by far our favorite. A few I wanted to see but didn't get to were Dunnottar Castle, Eilean Donan Castle. Maybe google some of the places I mentioned so you can see images of them and get an idea if they are of interest to you. Here is one of the better sites we used when looking for castles to check out: http://www.undiscoveredscotland.co.uk/uscategories/castles.html
and http://www.rampantscotland.com/castles/blcastles_index.htm
. Also, a majority of the castles require an entry fee which goes to help preserving /maintaining them. The only thing I didn't like about Scotland was once you get out into the country there are no shoulders for the road... so many times I wanted to pull over and take a picture of the scenery but there was no where to stop. Enjoy your trip, it's an awesome place!