I'm not sure I fully appreciated the beauty of this age range with my first two. It's the age of movement- rolling and sitting and crawling. Maybe even walking for some munchkins. They can finally move to get to that much desired toy instead of screaming their heads off for an adult to come help them. Or not help that as is the case in our house more often than I like to admit.
While we can't have a floor bed with our little one we can still provide an environment that encourages movement and freedom.
Balls are a huge hit. Especially one with different textures, sizes and shapes. We love the B. Oddballs (you can find them cheaper at target).
Bucket of matchbox cars.
Or a bucket of large wooden items with wheels if you are the type concerned about the previous idea's choke-ability factor. Your big kids may even join in on the fun.
Our baby friendly shelf with some large handle puzzles, shape sorters, books and a place for those highly coveted balls.
Space to move, something to move for and plenty of time is really all these little guys need.