The main one for me (and it's not going to be very popular on here I don't think) is ride Space Mountain.
That ride is single handidly responsible for my addiction to roller coasters, so I have to ride it at least once in a trip. Sure it's rough; sure they've "improved" it with the mission 2 overhaul, but I've still got a soft spot for it.
The other thing I do now is to walk round the entire park before leaving - starting in frontierland and working clockwise round the lands, before joining back at the castle and walking down main street - turning for one last glimpse of the castle at the end. That one originates from DL last year where I did that expecting not to be back any time soon so I wanted to have one last look at the whole park.