Hmm...more ideas?

NINJA: Black turtleneck (best) or long sleeved shirt, and black pants (sweatpants look best) and 2 strips of bright (neon) fabric. Use one strip for a belt and one for a headband. Black stretch gloves (about $1 at WalMart or Target) make a nice addition if you're in a cold climate. You can make ninja knives from cardboard.

STORMTROOPER: White sweats with black turtleneck underneath, white or
black stretch gloves, and white shoes. Make the helmet from a plastic milk jug. The handle of the jug would be the back and the bottom of the jug is the top of the helmet. Cut around and over the handle so the back is all open. Cut out the nozzle, so now you should have a jug with just the bottom and 2 sides. Put that upside down on your kid's head and mark where to cut out eye holes. Make sure the holes are large enough for your child to see out of easily, and you may need to make it all open almost like a motorcycle helmet for your kid's nose (helps to look at a stormtro0per to get the shape right). Paint the mask white with Odds-n-Ends paint, and for the black parts(except eyes) glue gun construction paper, felt, fabric, or use paint. If you want to make the belt cover small boxes (juice boxes work) or cut pieces of styrofoam then attach to strip of white muslin to tie around waist, tucking ends in. If you want eye area black like real stormtroupers use makeup.

DARTH VADER: use black sweats, paint the stormtrouper mask black, and add a black cape. Light Sabre can be made from wrapping paper tube. (there is a cheap one available at Wal-mart AND Target for about $5 too!)

PETER PAN & ROBIN HOOD: These two costumes use the same pieces almost, so a child can be Peter Pan one year and Robin Hood the next, and all you need to change is the smock. For Peter Pan get green fabric, fold in half, cut a hole for the head and cut the bottom jagged. Tie on with a belt made from a strip of fabric. I'd also add a pair of shorts to your list of costume pieces. In cold climates use green sweats instead of tights and forgo the shorts.

KNIGHT: Silver lame for smock, (Fold it in half and cut out head hole just like for Peter Pan and Robin Hood) white, grey, or black turtleneck and tights with shorts or sweatpants. Make a shield and sword from cardboard covered with foil. For more details you can embellish these with puffy paint or glue on plastic jewels. You can also cut out a coat of arms from felt and glue or sew to smock. Another addition is using a hooded sweatshirt or sewing a hood from the silver lame(about $3 a yd at WalMart)

DRAGON: Recycle the green shirt, tights, pants, shorts, etc from Robin
Hood or Peter Pan then add purple wings made from felt (can safety pin or baste on). Sew a tie-on green tail with a row of pointy scales made from felt, and glue some felt spots on it. Make a hood with more pointy scales up the middle, use a green stocking cap, add small horns to a headband, or just paint your child's face green with purple spots and skip the headwear. If you want your dragon to "breath fire" tape streamers to a dowel.

A FLOWER: (better for baby) Buy a sleeper or sweatsuit, add a flower. Glue silk flowers to a bonnet (or a stocking cap, but make sure the cap is stretched over a ball of newspaper approx the size of baby's head so there are enough flowers) and to a pair of socks that baby is growing out of so you don't ruin a good pair. For cheap flowers compare candle rings to the other silk flower bunches. You can always add the leaves in as filler if needed.

BAG OF GUMBALLS: Wear all one color of clothes, long sleeve shirt and leggings. Take a large clear garbage bag and cut two slits at the bottom of it for her legs, just big enough for your legs to go through without cutting off the circulation. Then blow up a whole bunch of different colored balloons. Stepping into the bag, fill it with the balloons, and had someone help you put armholes in. Then bunch the top of the bag around your neck and made it stay with some clear strapping tape (not too tight - that's why the strapping tape is nice - it holds the bunching in place, so you don't have to strangle yourself trying to keep the top of the bag bunched closed.) And then loosely tie a length of braided yarn around the strapping tape. Add a red beret, and tie a large price tag cut out of stiff paper to the little sticky-outy bit in the middle of the beret.