Many thanks for the replies here so far! I do have a lot of castles and other places on the map that I made for my in-laws. Where we go on any given day will be decided by them, and probably depend on weather and whim at the time!
If anyone is curious, here is the general outline of our trip. (If you aren't curious, just skip this paragraph!) We'll fly into Manchester on May 14th (having left here on the 13th), pick up our rented vehicle and drive down to Stratford-upon-Avon. I expect we will all be somewhat jet-lagged so that should be a fairly short, easy drive. After seeing some sights there, we'll stay outside Warwick that night and head to London the next morning, where they have friends. After two days there, we'll head on to Salisbury and Stonehenge (one day) and then into South Wales (as mentioned above). We'll stay in Cardiff one night, and in Kidwelly the next night, before heading north to our cottage near Dolgellau for the second week.
I haven't actually planned out the third week (after my in-laws leave) nearly to that extent - just (probably) entirely in Wales and (hopefully) with some opportunities to meet up with other SSiWers. So do please keep those suggestions coming!