Cornwall isn't very near Cardiff, though you American's wouldn't think twice about a 6-8 hour drive. The good news is that you have to drive over 80mph before the police bother on the highway. :cool:
Wales is a country in it's own right, it has a great coastline and loads of mountains. If you want lakes, the Lake District is about 4-5 hours north of Cardiff, and there are about 5 zillion beautiful lakes there, as well as mountain ranges, etc.
Locations in the Lake District, hmmm....
The lake district is very small, especially for you Americans. I'm sure some of your national parks are as big as the whole of England if that gives you some idea I would suggest starting with Windermere Lake, following up through Grasmere and Ambleside, heading North for Penrith but coming down using a small road through to Ullswater, from Ullswater head down and take the mountain pass back to Windermere. From there take the forest road through to Coniston Lake and circle it.
Technically that drive will take you no more than 3-4 hours so you get some idea of the size! I have of course left out the whole west of the Lake District, and the southern mountain passes.
If you want I'll map out a bunch of routes for you that will give you a full taste of the Lake District.
There are about a billion B&B's (Bed and Breakfasts) and a load of hotels, do a search for accomodation, Lake District for more details. I like staying in the Travel Inn at Killington Lake Motorway Services though I'm sure the B&B's are cheaper.
Eileen Donan Castle is usually portrayed as lit up. I travelled down from the Isle of Skye at 2am for 2 hours to get this picture, but I found out when I got there that the lights are only on during the winter. I wasn't going to give up so from a car park opposite I lit up the bridge with my car headlamps for a 30 min exposure. (I couldn't light up the main building because of the angle of the car.)