I travelled over a long-closed line only last October - from Douglas to Peel on the Isle of Man, plus most of the branch to Foxdale. Took me all day and very muddy it was too in places...
I also covet the IOMR lines which perished in the late 1960s -- to my great regret, I didn't have a chance to make a first visit to the Isle of Man until summer 1969 -- a year too late for Peel and Ramsey. Track was still down at the junction outside Douglas, and at Ramsey; but no more trains.
An interesting array of choices (even if we were only supposed to choose one, I thought )
Of course, if one could go back to 1965 or whenever, I'm sure ANY visit would be fascinating. But while, eg the Woodead route was unquestionably spectacular - Oxford to Bedford (why not to Cambridge?) was really not so special. In fact, Oxford to Bletchley was very hum-drum. Bletchley-Bedford was/is marginally interesting, I suppose. The most interesting bit was probably Bedford- Sandy - single line token changes, and more river views, plus the junction with and the bridge over the GN at Sandy. Sandy - Cambridge was nice enough.
Tastes differ -- I loved the Bletchley -- Oxford portion of the line, but found Bletchley -- Bedford rather dreary; Bedford -- Cambridge, for me "in between".
I never did it, but I suspect March-Spalding would have you falling asleep after two round trips.
I'm biased -- my early childhood was in the Fens (Spalding and environs), and I love the area. My very first years were spent within sight of the Spalding -- March line. Travelled over that stretch only once, in 1964: for me, a fascinating experience.