If you can't run the sump pump output to the storm drain (if there is one), then I'd consider a drywell (a pit with a perforated tank in it) - run the output there. Note, this should be away from the house, or you might just end up pumping it back out again! You want the water downhill so it doesnt' run back towards the house. You'll have a skating rink in a cold spell if there is regular water in the sump. You could also install a cistern, and use that water with a pump to water your lawn and garden in the summer when there is a watering ban - you might have the only green lawn in the area!
One of the pros may have some other thoughts...I would NOT just dump it out onto the lawn.