Wow, sounds like fun! I want to be a magical cement sidewalk with the power to do nothing.
Now that's funny! But if you were a magical cement sidewalk...you would have magical power! You could be solid for the "good guys" and liquid for the "bad guys". A bad guy steps on you and he sinks and is never seen again! But, unfortunately you'd have to play some type of character. It doesen't have to be humanoid either. You can also be a magical animal.