Big Thunder Mountain Railroad is an excellent roller coaster in Frontierland and one of the best attractions at Disneyland. The minimum height to ride is 40 inches.
This relatively mild coaster takes you through an abandoned gold mine in the old west. Try the "goat trick," by keeping your eyes on the goat as you descend the second hill for as long as you can. How mild is this coaster? The front cars of the train will bottom out on the third hill before the middle of the train even crests. A 2014 refurbishment added an "explosive" new surprise to that hill, though.