because you have to be fairly smart to make anything especially functional on gmod sandbox
I must be a dumb genius

I put my 2,808 hours into gmod playing sandbox 95% of the time
here's some things I seem to have fun doing in sandbox:
breakable houses (for gm_tornadohighway) or just crash a vehicle into it
destructable/crashable vehicles
build piston engines
a tall tower with a weld force limit (knock it down)
a semi or limo
tow a trailer on a jeep or a vehicle you've made
experiment with tools and addons
get on gm_mobenix_v3_final and race with bots or friends or build a train on the tracks
make a blender
build a base, disable AI and place combine or zombies and make a little mission to retrieve something or eliminate someone
make a crane or some sort of lift
build a transmission
build a vehicle with suspension
go on a space map and build a rocket or rover to send to another planet
I used to play the go-fish gamemode but no one has converted it to gmod 13 yet :(
or you could join a zombie survival server those are pretty fun.