Keep play sessions and walks short. You could take him places that you know are safe (as in, rarely frequented by other dogs - or to a place that you know the dogs are well cared for and up on vaccinations), and carry him around at first just so that he has some exposure to the real world. For example, when I had a little pup, I took him to our SchH club, since I knew that all the member and their dogs, but did not take him out to pet stores of public places until he was older and had at least a couple rounds of shots.
For the first few days, play it by ear as to how much to do. If pup is stressed and wants to sleep, let him. If pup is hyper and wants to play, take him out and play. He probably won't want to have anything to do with leash walks at first, so get him used to dragging a light leash around, and get him used to being walked around a bit in the yard with you holding the leash.
By the time he is 3 - 4 months old, he should be able to do 20 to 30 minute leash walks without any problem, other than getting bored (bring treats, a tug toy to play along the way). Though he should be able to do nice long walks/hikes off leash on soft surfaces, running and walking at his own pace.