I have found the following forms of groundworking really useful with my young horse...backing up, turns on the forehand and rollback, Going up through walk and jog on the lead, circling and changing direction. He has got to the stage now where he will do these things from hand signals and voice commands, or just following me, with no pressure, which will hopefully be really useful when I come to ride him.
I also find (because I cant ride him yet!) leading him out alone around the lanes really useful. Getting him working nice and relaxed with minimum pressure and maximum concentration is great for bonding and again reinforces voice commands.
Although I do occasionally lunge him dont really enjoy it, so tactile groundworking suits me best!
You could also try looking at the pat parelli 7 games
http://www.todayshorse.com/Articles/LanguageofHorses.htm as something different to do when you dont ride - variety is the spice of life!