The concrete by it's self will work better than concrete with plywood over it i think. The wood will still have some give and bounce.
I think you would be money ahead getting the granite to start with. You are going to have to get several 2x4 to brace your form with before and after rebar to put inside the top for strength. If you paln on using concrete that comes in the bags. It's going to ALOT of them. For instance if you dig a post hole 6 inches wide and 2 and half foot deep it will take one bag to fill the hole up with the post in it. If you order concrete i bet there will be a 2 or 3 yard price you have to pay even if you want 1 yard or a half a yard. Ot you can get the rock, sand and cement and mix it your self that's alot of work.
Have you though about going to a momument shop and seeing if they have a chipped stone or one that they have replaced with a new one. Another choice would be to get a babies tombstone. Their plenty thick enough and small enough you can move it by yourself.
My suggestion is bite the bullet and get a stone to start with.