That depends.
Most likely I'd just stare at it until it went off. Seriously, who WOULDN'T be shocked if a grenade landed in front of them?
Assuming I collected my wits in time, I wouldn't lob it back, as you might throw it right in front of someone else. I'd take cover behind something solid (if there was anything nearby), or better yet, jump in a ditch. My final resort if neither cover nor ditches were nearby would be to run for two seconds and lie down as quickly as possible, legs together, feet pointed at the grenade. Hopefully, any shrapnel would pass by me or at least miss my head/torso. If it didn't go off, in any scenario, I'd count to 60, to make sure, then call the police, while staying in the general area to warn off anybody who comes by. If anybody was nearby when I first saw the grenade, I'd yell "Bomb, GET DOWN!", in case there was somebody who didn't know what a grenade is.