I made this Baby Buzz Lightyear costume and didn't have to sew anything but the cap. I took an old sleeper that I had from my daughter that was pink and bleached it white. Then, following a store-bought Buzz costume in a bigger size, cut felt pieces to match. I glues them on with hot glue because I didn't have any fabric glue on hand. (I have an adversion to fabric glue--growing up my mom didn't sew, so she glued EVERYTHING. I swore I wouldn't do that, but its fine for anyone that doesn't have psychological damage from it)
Just about any pattern can be made into a Halloween costume. Its just the fabric that makes the difference. I made my son a Mad Hatter costume by taking a pattern for a regular fleece jacket and pants and used costume fabric from Joann's. For the hat, I took leftover fur from a Santa costume and glued it inside a painted plastic top hat purchased from the costume store for 99 cents.
For my Cruella Deville costume I continued using the remnant of fur from the Santa costume to make my dalmation stoal. I took a fabric marker and colored all the marks on it. Added my favorite black dress, a thrifted 50 cent belt, and a lot of black and white hairspray to complete the costume.