So many to choose! But I'll start with 2 nobody's mentioned yet: firstly, Cleethorpes to Retford on the Saturdays only via Brigg. The scenery through the Lincolnshire Wolds is very pleasant and the times are quite good to have a few hours in a lovely market town with lots of railway interest, or in the other direction for a day at the seaside. The other is the Island Line, in it's own way beautiful, and where my dear wife and I had our honeymoon!
Any of the Cross-Pennine routes are breathtaking with beautiful scenery and I never tire of them, I only wish I had time and resources to sample them more often!
The Cumbrian Coast is also up there for me but I think top spot for a railway day out goes to a Welsh scenic circle as follows: Llandudno to Blaenau Ffestiniog (Conwy Valley), Ffestiniog Railway to Porthmadog, Cambrian Coast line to Barmouth (break for lunch and pics of Barmouth Bridge), continue to Shrewsbury, loco-hauled to Llandudno Junction via Chester then back to Llandudno in the evening. I did that a few years ago as part of a 4 in 8 days rover there which was absolutely brilliant!