Fortunately, the boys always seem to have endless things to do on their own and can so easily fill the day playing separately or together. I often only need to keep an eye on them, without having to even sit with them for a big part of the day.
A recent hot favourite is playdoh. We revisited our stash after a hiatus and the boys love it so much that they can play for 2 hours straight, especially little chip who asks to play playdoh everyday. Marcus used to fiddle a lot with playdoh when he was younger and would make all sorts of food for me (see older post).
On a non-school day, Marcus draws from the moment he wakes and is almost always at the desk drawing and colouring. He can spent up to 4 hours straight (no kidding!) doing just that, fueled only by his own enthusiasm and passion. In fact, sometimes we need to tear him from his papers to brush teeth, bathe and eat. Even on a school day, he would busy himself drawing immediately after he got home from school if he is not playing Lego or reading.
Having a sibling who is so close in age gap means a constant playmate. The boys are good at entertaining each other throughout the day, so I find myself more of a "watcher" these days then a playmate with them. They don't need me to play with them, anyway, I generally don't. I tend to suggest games and give them ideas of how to play or what to play with next and they get on with it. I like to think that my parenting style has encouraged them to be independent in many ways, especially when it comes to independent play. It can only be good, to me at least, especially as it frees up time and effort on my part and it is training ground for valued traits like creativity and independence in the kids.