HIGHWAY: dress in black pants and long sleeved black shirt, use white or yellow tape to create road markings/lines on clothes, then sew/glue/pin on little toy cars and trucks and add road signs made out of toothpicks and paper.
ATOM: take 3 hula hoops and arrange them inside each other so they look like * from the front. Wrap tin foil around hoops to make electrons. Then wriggle inside so that two of the hoops are on each shoulder with the third circling you. Tie fishing line to shoulder pieces in front and back to keep it in place. Wear black turtleneck and leggings and paste construction paper circles with plus signs on them all over body (protons) and plain ones for neutrons. Make hair poofy and paint face silver if you like. Group variation: go as a molecule (2 hydrogen, one oxygen) with correct electron/proton combos.
ATOMIC: get colored balloons. One color for protons and one for nutrons. Inflate them to size of grapefruit and use a marker to put "P" or "N" on them. Tie them to string, christmas tree style, and run them around your torso. Additional: you can get glo-sticks and put them on strings, too. But spin the glo-sticks around you and then you have electrons! You can really impress if you can tell people what atom you are by number of protons you have.
SPIRIT OF BASEBALL 1994: paint your face like a ghost and wear a baseball jersey, cap, and carry a baseball bat. 1994 was the year-long lockout season.
COMMON COLD: wear big box painted like tissue box with "Kleenex" written on the side and real kleenex box on your head, held on by rubber band or string. Coming out of the head hole on the big box is white tissue paper. Wear plastic nose with nostrils tinted red and carry wastebasket full of 'used' kleenex. Underneath, wear pjs and slippers and do under-eye circles with make-up.
NIGHT SKY: wear blackbody suit/black clothing. Sew, tape, or glue on gold or silver stars and a moon. For added fun, use glow in the dark stars and moon.
SUN & MOON: one person paints their face and hair silver and paints two big cardboard cut-outs of the moon silver and wears them like a sandwich board, with two straps over the shoulder. Wear black or navy clothing underneath. For the sun do the same - cut out a sun and paint it, your face, and hair orange or gold. If you have a trio of friends, one of you can be a star.
FUTURE SELF: go as yourself 30, 40, 50 years in the future. You can be literal and dress as what you'd look like as an old man or woman. Or you can dress as what you'd like to be in a funky space age future. Example: go as the World Wide Director of Serious Phun, an intergovernmental agency working to prevent war and conflict through fun. Wear a world map shower curtain as a cape, deflated globe as a hat, Groucho Marx nose and glasses, funny clothes, etc. Use your imagination.
BEETLEJUICE: the movie Beetlejuice has tons of great costume ideas.