My number one best tip for keeping your kids playing happily is: fewer toys!
It may seem counterintuitive, but honestly the fewer materials they have access to, the more deeply engaged they become. As an example, my two and a half year old somehow acquired about eight duplo blocks – a few squares, a few rectangles and one longer platform, and a window. These eight blocks have probably provided well over twenty hours of entertainment in the past few months. I have been blown away by the variety of configurations possible, the stories he tells about them, how he uses them to interact with other toys and with other humans.
Conversely, I have noticed that when confronted with a room full of fifty different options, the tendency is to spend five minutes dumping everything on the floor, and then be done “playing”.
(Update: it’s official! Science is backing me up on this.)
It is for this reason that I recommend rotating toys. Any time your child acquires multiple new things at one time, put away most of them in a hidden closet, and keep out just one or two to really explore. Then a few weeks or months later (depending on their age and how quickly their interest begins to wane), put those ones away and bring out a few new ones.
I bet you will find that their attention span for playing independently becomes longer. An additional bonus is that there will be far less to put away at clean up time!