For me there are several:
264 Forklift
497 coal loader/unloader, use small round pellets or beans instead of the real coal
362 Barrel Loader
364 Log Loader
464 Saw mill
352 Icing Station
345/346 Culvert Loader/Unloader is great but can be tempermental.
Accessories that are very visual tend to impress visitors.
Most of my visitors do not know the difference between a GP7, FM or a SD40.
Having accesible buttons that they can push ceratinly enhances their experience.