Make sure its dog poo as they could DNA trace the poo back to you if its your own

Theres tons you could do, cress their carpet when they are on holiday. egg/fish in car heater vents, have thier mail redirected, report them to local crimestoppers for terrorist activities after youve dumped loads of fertilizer in thier garden, sugar, a few assault vests (ideal to convert to bomb vests), wiring, simple timers etc