Quad biking is so much fun and with no shortage of mud, mountains and rivers in Scotland, it is sure to keep you entertained. It is also one of the few activities I can think of that is still as when it rains. There are many companies that offer quad biking and there are places all throughout Scotland. A lot of the places that do quad biking also offer clay pigeon shooting or archery too so it can be bundles together with other activities.
As well as mountains and rivers, some of the tours include time riding on sand dunes which is epic. It is possible to have a full day tour which will include a stop for lunch, some exploring and what is often referred to as dune bashing (driving fast on sand sand dunes).
It is also a good way to see some of the huge Scottish Estates or Hydro (Spa hotels) such as Peebles or Crieff. Places such as Crieff Hydro offer quad biking throughout there grounds and even have a Spa to help get clean after all the mud.