I have a three month old. He loves holding and eating toys, or sitting in a bouncy seat. We smile at each other a lot, and I talk to him.
Everything is really new for him, so he doesn't need a lot of stimulation. I really try to keep stuff mellow. They're picking up everything anyway. I does love interacting, though, so we make faces and talk and stuff.
But if he's happy "playing" by himself - looking at something, listening to music, mouthing a toy - I let him. They need to develop a sense of independence, even this young, and know that they're okay on their own. I'll do my things around the house, in the same room, and come back to smile at him, or I'll say his name and talk to him while I'm working.
Isn't this age just full of new, exciting discoveries? He's all of a sudden become so responsive!