We have two closets in the house that are filled to varying capacities with out-of-rotation toys. The boys' closet is where we house board games, small bins of different 'kinds' of toys, and my craft supplies. Here's what toys are currently in there, and out of daily rotation:
(each * indicates a single tub or bin):
*small plastic animals and My Little Ponies
*a few items of dress up to rotate in and out
*trucks and cars
*Barbies and action figures
*toy tool set
*pirate ships (you have to have two of these, or who will a lone ship battle?!)
*wooden castle and accoutrements
*army men, soldiers, knights
*puppets and puppet theater
*train tracks and trains
The bigger items, like the trains, castle and pirate ships, are toys that I'll pull out for a day or so, to give the kids something novel to play with. They're really great for when friends come to play, since everyone gets really into playing with an unusual toy, so they all stay really occupied and don't fight a ton. The smaller bins will come out occasionally to either join or replace the more semi-permanent bins the kids already have in their rooms.
Rocco's room also houses toys, but these are more of the 'puzzles and activities' sort that come out to be used at the dining room table, under varying degrees of supervision, depending on the kid.
(Yes, it's a hard closet to photograph, since it has a mirrored bifold door, and yes, if you're trying to count at home, we own three - count 'em, three - pack and plays. And have a huge stash of diapers, clearly.)
To some, it may seem like a lot, and to others it might not seem like much, but it's a manageable amount for me. I feel like it's enough to keep the kids' interest without overwhelming them - or me - with outrageous toy chaos. I simply try to limit the space I'm willing to give anything - a certain number of shelves for books, a certain number of tubs for toys, a certain amount of space in the closet, etc. Once it outgrows its space, I start chucking stuff. It's one efficient way I've found to keep our things from outgrowing its welcome and/or ruling over us.
As for kids' books: I am very, very picky about what kids' books we own, especially board books. We love Sandra Boynton and Eric Carle, and almost anything else is kind of open to the chop, should I so choose. For picture books for the younger set, I keep anything with stellar illustrations or stellar writing, and once the dedicated shelves are full, anything else is at risk of the chop. We have an excellent public library here, so I have felt less and less need to 'own' throughout the years, since we have basically any title we'd ever want available to us. And I have noticed that, with fewer but better books on our personal shelves, it gives the kids the opportunity to fall deeply in love with a few excellent books instead of just showing mild interest in a lot of mediocre books, if that makes sense.
The lower two shelves of that wider brown bookshelf on the left are where I keep the kids' books: picture books on the lower shelf, chapter books on that divided upper shelf. Those are all the kids' books we own that the kids have free access to. We use the library heavily, and there are many classic titles that are kept on upper 'school literature' shelves, so these are their free reads. Everything else you're seeing is either books for school, or books belonging to me and Todd.
I lean heavily on the Ambleside Online booklists, and the 1000 Good Books list to guide what I keep. Honey For A Child's Heart is another great resource for finding high quality literature for children of all ages. I don't religiously follow these, but they really do offer great insight on the authors and titles that will feed the minds of our kids in the long run, and help me discern whether a book is a keeper or a tosser if I have to start weeding things out.
And that is how we keep our kids' stuff organized, limited, and easily accessed when we need it!