Suggest Prestwick or Cumbernauld to Oban or Glenforsa then up the Great Glen to Inverness or Dornoch. If you have the time, Orkney is well worth a visit although apart from Kirkwall, some of the strips might be a bit short for a fully loaded four seater. On the way back south, suggest you visit Easter and Perth or Glenrothes (nice restaurant on site) before flying past the Forth Bridges, the Falkirk Wheel and the Kelpies.
Of course, you can do the trip in reverse if the weather favours that.
Back in the south, a trip to Islay is well worth it particularly if you are interested in whisky. Also Castle Kennedy with a Castle and nice gardens nearby. Plockton (perhaps from Oban or Glenforsa) is a lovely place to visit on the NW coast but you can easily get stuck there!
Finally, if you fancy landing on a beach, consider a trip to Barra although it is a fair amount of water to cross and some clubs might not allow you to land on the (salty) sand. Perhaps you could consider a return flight in the Loganair Twin Otter from Glasgow. On a nice day, they often go VFR.
Do not hesitate to pm me when in Scotland, if you want further advice or help.