My DS's (6yrs and 2yrs) are both very busy boys, and both have the attention span of a gnat!lol! so when its rainy, or i have a lot of housework etc to do, meaning im gonna be stuck inside for a good part of the day, I need to make sure that they are kept busy (they can destroy a room in under 2 minutes!lol!), so we do things like:
play with bubbles out the garden (to avoid slippery floors!). fairy liquid and water mix will do, save having to go out and buy some, and you could twist or tie the ends of pipe-cleaners or plastic straws etc to make blowers.
play-dough (a life saver!) again could make your own, recipes for it online are pretty simple :)
painting, drawing, sticking etc
they help me to cook and bake
fill up a tub/bowl with water, grab some waterproof toys, spoons jugs, old bottles etc and either pop it out the garden or onto a good layer of towels, and have some water play
make a tent with an old sheet :) my boys LOVE this!peg it to some chairs or other items of furniture (just make sure nothings going to topple over on him if DS pulls on it) and play camping. my two usually have theirs up most of the day and insist on eating their lunch in there etc.
If you have an old cardboard box around thats big enough for him to sit in, decorate it with him as a bus, digger, rocket, submarine etc etc and let him play in that
my two have excellent imaginations,so if im not busy myself, i play imaginary games with them, like Tree Fu Tom, pirates, doctors or police men etc where we all act as a character or i put a throw blanket on the floor,put cushions on top of it for our seats and then we use a plastic plate as a steering wheel, potato masher as a gear stick and we "drive/fly" around (depends on whether its a car,rocket,plane etc this time) and go on safari, into space, under the sea etc etc and point out all the things we "see". We even get chased away by a rogue elephant, swallowed by a whale or have to battle enemy aliens on some trips, you never know whats going to happen! ;) lol!
hope a couple of these will help :)