Heh, nice!

If there's hooks already you can put pictures etc up. If not check with the r/e as some are fine with it others say no hooks, no 3M removable hook thingies or anything. Carpet rugs are a good way of disguising icky carpets, putting up some nicer curtains should be ok so long as you can return the existing curtains to their rightful place at the end. Depending on the age of your kids, little ornaments and things or vases with silk flowers can add some interest without breaking the bank.