Well in this household, June-September is party season, as a lot of kids we know have birthdays during this period.
There were 2 parties recently that required costumes - firstly, a space party, where my son dressed up as an astronaut, and my daughter an alien! Astronaut costume is a grey tracksuit, plus packaging painted white with detail and attached to a back pack to make an oxygen tank thingy, with a helmet made from paper mache moulded from a balloon. Full credit to my husband for making this costume with my son helping. Alien costume was a green dotty tutu plus shrek ears (both purchased).
Finally, last week, there was an Arty Party, and our son went as an art critic! Everything was sourced from our own wardrobes - black cap, skivvy, & black pants - the only thing we did was "colour in" black a pair of unused orange kid's sunglasses, and remove the lenses.