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I'd go for the string trimmer turned into a gas powered blender. Girl will be very impressed when you whip up some strawberry daiquiris out of no where.
Something at least mildly dangerous.
Moped a bicycle. L
Put it on the back of a skateboard or (stand-up) scooter? Build a go-kart?
Build a giant scale R/C plane. Put that on your bike instead of the 25 cc. You're still gonna get hurt cause you won't be careful enough though. Gocart.