Obviously the West Highland Line and the Kyle Line are up there, but I'd recommend the Far North Line after you've done Kyle. It has everything - the Beauly Firth between Inverness and Dingwall, the Dornoch Firth which you follow all the way inland, the bridge over the Kyle of Sutherland into more rugged, sweeping scenery (apparently you can see the likes of Kinlochbervie and Laxford Bridge when it is clear enough), the coast between Golspie and Helmsdale (for most of the time between Brora and Helmsdale it seems as though you're actually on the shore), then the marshland north to Forsinard and the barren, peat bogland (keep an eye out for deer!) from there to Georgemas Junction, Thurso (where I'd very quickly jump off and on again to say you've stepped on the UK's northernmost railway platform, then take a picture of the sign through the open door) and Wick. Sometimes there's a trolley as well.