RE: What retracts?
I agree that mechanicals would probably be the best way to go. I have a set from Supra that I have been using for many years, but I don't know if they are still in business. B&D and Dave Brown also make good units.
Make sure the mounts are very strong, as grass is hard on retracts! Also, check them after every couple of flights to make sure they haven't bent so much that your wheels get stuck in the up position (been there, done that!).
Good Luck!