Those Plants which grow out of pipes on long snaking vines: A defender that returns to hand when damaged.

The Squirrels from 1-1 of one of the Mario games. They carry squirrel suit powers, so make let them give another creature 1/ 1 and flying?

The boot as a possible equipment/vehicle.

1-up mushroom: Probably just a life gain?

spikes/fire bar: Attacking creatures take x damage?

Doors: Your creatures can't be clocked this turn?

Key: Maybe an equipment? When you would sacrifice one of your artifacts, sacrifice this instead? No real clue how else to get it to 'combo' with other artifacts.

Red coins: Maybe some sort of token? It should be able to be sacked for any mana like the treasure artifact "coin"

Secret exit: Removes a minion from the battlefield? One of your own, with an additional benefit?

False wall/Hidden Room: Should be ghost house, maybe just card draw?

Spawn pipe: Create a minion token every turn.

Checkpoint flag: Allows you to tap mana to regenerate a minion.