Condoms on door handles.
Plant a weed plant in their yard and then call the cops.
Put porno mags in their mailbox
Silicone lube on all the door handles. They won't be able to turn them.
Paint over the gauges on their gas/water/electricity meters. The power company won't be able to read them, and they'll get fined.
Baby powder over EVERYTHING
Turn their yard into a putting green.
Plant a Nazi flag somewhere
Steal the lightbulbs for their outside lights
Oil. Lots of it.
Bleach on their grass in any shape you choose, preferably a penis.