With DS1 we did lots of groups in the mornings (mother and toddlers, tumbletots (exercise/gymnastics class), sing and sign, soft play centre), and then visited friends / went out with friends in the afternoons. We were so busy, that an afternoon in by ourselves felt like a nice rest!
Now we have DS2 (21m), DS1 is in pre-school 5 mornings per week. That's a great place, and he gets to do lots of arty things there. DS1 and I don't do any groups, but he seems fine. We do something most afternoons though, whether it's a trip to the park, duck pond, soft play, aquarium, playing out on the street etc. In the mornings we go shopping, or come home after dropping DS1 to p/s and do housework. He has a toy hoover, brush & mop, so we do that sort of thing 'together'. Then he's usually around me while I do other things like washing, or putting clothes away. I quite often spend a couple of hours sorting out the boys bedroom whilst DS2 plays happily by himself in the same room.
I don't really spend any time playing cars or trains with him like I did with DS1, but DS1 plays with him, so I don't worry about that. I will join in if it's something they need help with (marble run), or supervision with to stop them fighting (intricate farm set). If it's something like building blocks, or duplo, I try to leave them to it.
If you're feeling bad about it, try the 10 minutes thing. That seems like a really good idea. Or just spend 10 minutes tidying up one activity & encouraging him to move onto the next one.
It gets easier as they're older / you have more children. Now DS1 is nearly 4, he quite often goes to play at a friends house by himself! If I can time DS2's nap to coincide with this it's bliss I can't wait until the summer, as I'm hoping that this year they'll be old enough to play out in the garden together by themselves.