Last weekend I built a simple rollercoaster together with my youngest son (he's 19 now!). It's nothing compared to the big coasters some others have built, but we did have fun!
It only uses 'plain' Lego, old 4.5V train parts, standard bricks, so nothing fancy like loops etc.
The trickiest part is having enough speed to climb, while still going slow enough not to tip over in the curves...