I'm not into vandalism, but there's one bit of mega-grafitti I've always thought would be fun.
Choose a stretch of motorway between two junctions. Get a tractor, and fit with one of those crop spraying things with arms that come out each side - but you have to be able to cut the flow off to the right hand side. Fill with green paint.
Drive tractor onto motorway in dead of night, and spray the hard shoulder green. Meanwhile, the dozens of mates you have helping will dash out from periodic hiding places and stencil bike symbols on the green.
Of course, CCTV means you'd be spotted before you'd got going, but I like the idea of early morning drivers being faced with a motorway scale bike lane....
Better still, turn the hard shoulder over to cars and let bikes use the whole carriageway. Service stations would eventually become enormous decent cafes, with a single petrol pump outside for the token driver...