save the money from the tile/concrete and just remove the grass and level the yard really well. If you are indeed talking tile, that can get really slippery when wet.
I have a good friend who has a very level 15 foot dirt circle on her yard, outlined with stones... They round-up the edges and put up the intex every season on this same circle. Looks a little odd in the winter, but a tiled yard I think would look different, too!!