Hi, I wouldnt' worry about 4hr EASY just yet, I know it's something you want to strive towards, but I really didn't think about actively extending out activity time until my dd was over 3mths. At this age, as long as you're following your ds's cues for when he's tired that's the best thing you can do. Be guided by him for now, especially if what you're doing is working - if it ain't broke, don't fix it. When the time comes to stretch him out, I'd do it gently and in a low-key way - maybe walking around the house and talking about what you see.
As far as what else you can do, have you been doing tummy time on the floor (a hard surface is preferable, but if he's adverse to that at the moment, anywhere is fine).
Looking in the mirror together.
Just observing daily life is fine for now, he probably won't be too interested in toys just yet, but just hold them for him to focus on.
I used to sing songs with actions (ie wheels on the bus and I'd move my dd's arms in circles, out and in, swishing etc).
You can do cross-patterning which is where you have your ds lying on the floor on his back and cross the opposite arm and leg and touch together (it helps to programme for crawling later).
HTH