I'm going to follow this with interest...........
my 3yr old is really not that bothered with his own toys (usually enjoys anyone else's!) although i have noticed that he has more patience with some of them recently so there is hope

liking Anna's ideas.........
hard to think of many other energetic indoor things........Louis prefers non-toy toys so i've acquired various things such as a windup torch, binoculars and role play toys - he likes cleaning with his dustpan and brush

standing on a step stool to 'wash up' at the sink. play doh, painting, chalking, magic maize. scissors are a big hit here too!
in desperation times i have given him a bath - even in the middle of the day! - and let him take his water shooter in there. we also blow/catch bubbles in the bath
have you considered a little trampoline for indoors?