We converted our double garage into a playroom but kept the back of both for storage. So the playroom is 5.4m long x 3.6m wide.
We have windows on one wall (the 5.4m one) and opposite I banked it with a 4m run of PAX wardrobes from ikea.
There is a massive sofa bed which is opposite the wardrobe with the tv and wii in, and there is also all the games they can play without me needing to be with them so connect 4, dominoes, snakes and ladders etc.
On the top shelves out of reach I have playdoh and even my almost 9 year old still plays with this. Also paints/glue/sticking stuff and anything else I need to be with them for, like scalextric.
We also house all coats, shoes, hoover, ironing board etc in these wardrobes.
We have Ikea Trofast storage for pretty much everything else as I bought it years ago. So back then it contained a massive amount of wooden train track in the large ones. cars, k'nex and dress up in the medium ones, paper, felt tips, and the glue etc in the small ones.
As I have more tubs than storage I used to swap them out so they wouldn't play with the same things all the time.
Basics are
storage and more than you think you will need
Place to sit for you
Table and chairs for the children
Space to do jigsaws, cars, trains, run about