I've just had this situation with ds who's 3.5, dh was away for three days and I was panicking that being 34 wks preggers and fed up would make it a nightmare time! But it wasn't so bad at all, we had a great time in fact (although I acknowledge that at least there's only one of him whilst you've got two to contend with)
We took a bus ride (he's not been on once since he was tiny so this was v exciting)- not an at-home activity of course but one that necessitates sitting down for you
Made a wormery out of an old pop bottle, soil/veg peelings bit of water and of course went worm hunting for worms in the garden
Made a treasure hunt, hiding "treasure" (circles of card covered in foil which he made first, two activities in one- bargain) round the house for him to find
Yes to the swimming too (again of course not home based this could be a treat for you too, just being in the water and weightless will be lovely)
And get a friend with similar aged child/ren round for a playdate! Preferably one who isn't pregnant as this person, if a true friend, will not allow you to be the one who fetches juice/makes snacks/sorts out squabbles etc etc and may even bring cake . My lovely friend did this on Friday and I could have kissed her for being so kind, it was so appreciated.
Have a fun couple of days!