Our baby sleeps in his pack/play whenever we're at Grandma's house. If your baby has never slept in it before, I would try to get him to take at least 1 nap in it before going on your trip...also in a different room than he's use to sleeping in. Essentially, you're training him to sleep somewhere other than his room and his bed.
Yeah, it's probably not as comfy as his mattress, but I don't think putting foam or extra padding in it is a safe idea. Your baby will probably protest it b'c it's out of the norm, but hopefully he'll settle in for the night. Also, let your baby play in it maybe 15-20 minutes per day as well to become even more comfortable with it. At first, our baby had trouble and cried b'c it wasn't his crib; but, now I think he knows that when he's at Grandma's, that's his bed.