I was a SAHM when mine were at school (now older). We used to do absolutely nothing at all after school when they were little. By the time we'd walked home, there was time for pottering, drawing, etc, then children's tea, bath and bed. But it was just part of the daily grind routine. Once DC1 started school, I had other little ones at home, so 'after school' wasn't anything special (though the routine of having to collect DC1 filled up some time usefully every day). We didn't do anything special - no activities or anything. There weren't Ipads or phones then, and we didn't have a TV. I took them to play with school friends every now and then, if they were local enough (lots weren't), which we all enjoyed.
In your situation, OP, I think I might have been tempted to work three full days (would never have imagined saying this when I was a SAHM to little ones, but giving up my job turned out to be a mistake!) That way, you can catch up with some stuff on your two days off, and collect the DC on those days. I know it's nice to be there to collect them, and SAHMs were the norm at my DC's junior schools (so I'd have felt a bit out of it if I hadn't been there at pick-up every day) - but you do just need to do what works best for you and your family.
My final thought is that this kind of 'non' time is, I think, much easier if you're used to being with your DC 100 percent of the time. I was with mine to that extent, and there's only so much entertainment you can provide. Especially by the time 3.30 rolls around.