I've got a few things I like to keep an eye out for.
The North Clyde was my local line for many years and I commuted along it almost daily for five of them. So I've become quite intimately acquainted with parts of it that I like to keep an eye out for. The old junction for Bridgeton Central leaving just to the East of High Street can still be seen, as can the old line leaving just to the East of Shettleston (heading towards South Lanarkshire). A few non-railway related things too, like the abandoned trailer park near Carntyne - I became so familiar with the line that I could tell if someone had moved a bit out or added it in! On the way to Partick, you'd have really good views over the Clyde and occasionally you'd see something quite different as well - the waterplane landing on the river was probably the best, but you would occasionally get a new small ship. Laterally, the South Glasgow University Hospital taking shape in the distance was interesting to watch, and of course I'd have to look out for my old Uni to the right!
Going down the M74, I think the lollipop/mushroom is quite a significant landmark for lots of people in the Central Belt. I won't repeat what we called it coming home on our school trips - I'm sure you can work it out!
I quite like going past the airports at Aberdeen, and particularly Edinburgh. I'm really hoping to have a plane pass right over my HST on its final approach. Sadly, it hasn't happened yet

But you can see some interesting traffic at both airports, moreso at Aberdeen during the week with a plethora of helicopters mingling amongst the turboprops.
Angel of the North is a must is a well when I'm on the ECML.