We went with a Dora and friends costume for Halloween. It was a good time. Dora, Boots, and The Map had a blast.
The Dora costume was probably the easiest. We just found the appropriate colored items of clothing and purchased a wig from a local Halloween shop. $40 total.
The Boots costume wasn't too bad either. We just got some tights and leotards and sewed the yellow chest pieces and tail on. The ears and hair were sewn and stuffed by hand around a headband. The tail was also sewn and stuffed by hand and we used a few pipe cleaners to maintain the shape. We got the boots of ebay. $35 total.
I actually got the idea for the map costume here...
I bought a foam mattress pad off eBay. The hardest part was measuring and cutting it. I rolled myself up in it and measured and re-measured multiple times, before I finally cut.
Next I cut the arm holes. I measured multiple times for the arm holes, as well, wrapping myself several times. Once all the cuts were done, I covered the foam pad with a cream colored sheet and attached it using a glue gun and spray on glue, being careful to keep any visible areas free from glue.
I cut the sheet at the arm holes from the center using four cuts being sure to fold and glue the excess to the inside of the costume. I then did the measurements for the final wrap so the costume wouldn't be too tight or too loose. The glue gun was used again to hold the wrapped shape.
I had trouble keeping the costume up on my shoulders so I attached a set of suspenders to the inner folds out of sight for better support. Next I used felt fabric to create the eyes, mouth and facial features. The foam pad was probably the most expensive part, but it was worth it.