Great suggestions. We are probably looking at sometime this Autumn with a careful eye on the weather. Pen-y-Fan looks good and could be incorporated into a road trip. Before kids we used to make use of our proximity to Gatwick and grab cheap flights to Inverness before hiring a tiny car and ploughing down the length of Loch Ness heading for the mountains which could be an option too with a bit of forward planning (and luck on the flights).

Another +1 for Cader Idris, either main approach although Pony Path is probably the easiest to the summit.

Also maybe consider Fan Brycheiniog from Llanddeusant, a great ridge walk.

Here