Thin pickings on the southern section, but:
In Cambridge, just after you've taken your standard tourist pics outside King's College Chapel, don't miss the time eater clock on the left on the corner of a Bene't Street.
About 25 km after Cambridge, Audley End country house on the left.
About 15 km before the end there's a random hump backed bridge for Blake Hall station, a former London Underground station in the middle of nowhere serving absolutely nothing.
Barely a sight, but northbound about 75 km in there's a tiny marker post on the left where we cross the Greenwich Meridian. I'm surprised we only do this once, despite most of the southern section being very close to the line. I don't know if there's an equivalent marker on the busway on the southbound leg - if there were it would be a the road crossing (of "Over Road", of all places) just before the Swavesey bus stop.