Just head out on Sheffield Road until you see something appropriate, I remember the best spot being on the right side of the road as you head away from the highway, it was past the usual drinking and shooting spots, but there are plenty of them out there.
Near the pond at the golf course has some good ones too.
I don't know if it is still legal sled there, but there was a great hill out at Woodruff park. With a toboggan you could go from the top of the hill almost to the highway. Be sure and get permission if you need it.
If you have an old runner sled, the ice hill at the skating rink is fun. We were out yesterday and you could go all the way from the top of the ice hill to clear across the skating rink. Snow sleds don't go anywhere near that far, but still are fun. Located just behind Denton Field next to the tennis courts, probably the closest spot.