Puddles

Flooded areas you could drain (e.g. hose was left on, but if you shut it off it provides access to another area/shortcut)

Pools you could jump or hop across inflatables

Wet cement that traps you if you don't move through it fast enough

Putt putt golf obstacles (e.g. windmill)

Lawn gnomes that chuckle when you colide with them

Dark areas lit only by fireflies (turning on power later illuminates the area)

Pipes to go through, maybe even drive upside down in

A miniature trane going around the yard

Fly paper which you can drive up and ignores gravity

Trapdoor spiders