I assume your rabbits are indoors? Indoor rabbits can typically get all the exercise that they need in an indoor, bunny-proofed area. They are used to and are comfortable being indoors so that is where they are most likely to dash about and binky.
Outdoors can be an occasional outing, but that should only be done under your constant supervision. Otherwise, you would have to create a pen that has mesh wire both under the area (to prevent digging) AND over the area. It would need to be completely covered on top (to prevent predators from getting in -- including birds of prey).
Rather than messing with all of that, it would be easier to just get an x-pen or two and set that up temporarily outside whenever you want to take them out. This would be when they would need to be constantly watched, with you close by, to keep them safe.
Of course, this also means being aware of other dangers. The grass can't have fertilizer, weed killer or pesticides. You won't want bunnies to get ticks, mites, or fleas.
Indoor rabbits that are brought outdoors on occasion will usually do more exploring than actual 'exercising.' The attached shows how I've occasionally let my rabbits outdoors. I prefer to keep them off grass and all the potential nasties grass may have.