There isnt many outside of the park but there is a few inside the park itself.
Theres the Alton Towers sign in one of the cabins of the Magic Carpet ride at M&D's
A classic is the old "Aqualand" sign viewable from the river rapids queue line.
The old Planetarium projector is now themeing in the circle building in Oblivion's queue which can be seen here
at around 1:51.
There is parts of the old Alpine Bobsleigh somewhere in the woods behind Dark Forest.
Viewable from Th13teens queue is old concrete walls from the past rides of Ug Land and Festival Park such as Boneshaker.
In the gardens is an old abandoned toilet buidling which is filled with old (but dirty) artifacts such as cups.
Im sure theres also many signs which still have rides such as Dynamo still on them.
and sort of related in an opposite way and posted in another topic, the mushroom cloud luxury tours bus when it was still used as a bus.