The best advice I can give you is experiment. You'd be surprised at what you can do with few credits and buyable furnishings. Be creative! Crates, tables, posts and ice are great to give things height. Rugs can be used for walking, dancing and adding color, too. Most any room can be made up for Halloween. The new dark room would look awesome with pumpkins and sand rugs. Use trees, any trees for background filler. Or try something totally different! So many options. Try placing different things in a room and see a room take shape in your mind and go for it!
Most of all, have fun building!