Llantwit Major is worth visiting for a couple of hours at least - if only to see the double church used as a centre of learning before universities existed.
Quite easy to spend a couple of cultural days in Glasgow - visit Hunterian museum, Kelvingrove or Mackintosh designed School of Art or perhaps the Burrell collection in Pollock Park - I enjoyed Glasgow immensely. The Germans who visit the UK tend to be quite interested in culture even if some Brits are not !