Okay for painting wall of course do this without the birds in the house and let cure and air out for as long as possible. Use childsafe paint and should do wonderful. For your entertainment or possibly theirs you could add leaf stencil designs or jungle murals etc.
Toys -- I would suggest a boing hanging from the ceiling and or a nice swing with toys suspended on it (try to make sure that the birds will be easily reachable even if on the boing or swing or they can be almost impossible to get down if they are in a "mood"
, a large playgym with lots of toys hanging on it, a climbing net can even be hung on one wall with wood, acrylic etc toys hanging all over it. If you have the option of redoing he floor go with something easily cleanable, hardwood, tile or vinyl flooring. This will keep the dust down and make it much easier to clean messes.
Make sure your fish tank has a full cover so no birds can get into it.
You could even make a playarea for your hamster in one corner or side of the room with a "playpen" type area and chewable toys for him/her. Although be careful not to have your cockatoo and hamster out at the same time as there have been many accidents with large birds and small rodents, mammals and even smaller birds. A very nice tame bird can still follow its instincts and attack a tiny animal. Also even though I am sure you already do make sure to supervise your birds out time if you let them out together. Even if they get along well a playful act can turn into a sad ending when two birds are so different in size.