I have Shenzi sit stay while I walk around her (I use a training lead about 7 metres) And I will periodically drop a treat. When done, I stop randomly and tell her to 'find it' Once in awhile I'll bark a stop, and this command expects the dog to completely stop what shes doing and sit. Sit, walk 7 metres, come, half way to me, stop. Come again. We work on heeling exercises as well. If you have the money for a dumbbell, you can work on dumbbell retrieves as well. There's a lot of things you can do in an open field, put your imagination to it!