For my son, I made suspenders out of burgandy ribbon, and a bowtie and fez (fezzes are cool!) out of felt. Then he just wore his sunday shirt, a pair of blue slacks, and carried his awesome LED sonic-screwdriver flashlight.
We went a similar route for my husband, donning a tweedy-looking blazer from Goodwill instead of suspenders, and a cowboy hat instead of a fez. (I wear a stetson now. Stetsons are cool.)