Actually, fireworks + many thing would be fun. Chainsaw + Fireworks = Randomly spinning mess of death.
I also have a few combinations i would like mentioned.
Wheelchair + Battery = Electric Chair
Wheelchair + Lawnmower = Uhm, Wheelchair cutting zombies apart?
Diamonds + Flashlight = Awesome light saber
And one of my favorites: Plates + Cement Saw = Really deadly launcher, especially when there’s a lot of zombies. It can also give you A LOT of xp, and it shoots very far.