Garry's Mod is so fun!! I've just started playing it with some friends and trying to build flying hovercraft is tons of fun (and super hard). Building machines would take a lot of work but could be done. There are options to bind things together and whatnot, but no I'm not really sure how to do it. A very fun thing is an mod (?) for Garry's Mod called Monster Onslaught V2 in which you build a fort and hit a button and then monsters keep coming faster and faster until your inevitable death. It's a lot of fun to play with friends.