My DCs are nearly 4 and 6. We do 1/2hr YouTube exercise followed by 1hr total on maths and English each morning (even through the hols to give some structure). Then 'free play' until lunch. After lunch we'll often have an activity or walk (plus another 1/2 - 1hr school work in term time), then free play again until tidy up time at 5.
Like a PP, I will happily do activities/lessons/read books/walks with them or help them to set up a particular toy, but I'm not playing with toys with them. They get plenty of time with me, but need to know how to occupy themselves. They were not great at playing independently for a long time, and the older one still struggles sometimes, but they have learnt how. I found leaving them to get really bored worked a treat, although I had to put up with a lot of moaning whilst they got the hang of it! On the rare occasion they complain now I tell them they can either find themselves something useful to do or I will. That usually gets them scurrying back to their toys 
Examples of activities we do together are Hama beads (I think I enjoy this more than the DCs...), board games, Top Trumps (DC1), painting, craft kits, science kits, baking, bath bomb making, nature documentaries, films, making dinner.
Examples of things I set up and let them crack on with are cut and sticking, colouring, play dough, some board games (like the noisy fishing one), lego, playmobil, marble run, garden chalks. I also pitched our small tent in the garden at the beginning of last week, and they loved it so much we've just moved it to a different spot this week. They will often now make their own games up with their dolls and play kitchen or Sylvanians, and once a week will practise and put on a dreaded 'show'.
OP I'm an only child to a single parent, and remember happily spending hours at that age playing by myself whilst DM was busy although I also remember pestering her too sometimes. Fast forward a couple of years and I was very happy to play for hours with Lego and Sylvanians. Entertaining yourself is a learnt skill.