This is late for you, but others will benefit. There are lots of inexpensive places to eat on both islands. For local flavor, try an L&L Drive In. Get a lunch plate and feast! Just ask any local where their favorite place is to go eat.
For shopping, go to the Aloha Swap Meet on Oahu (Wed, Sat, Sun. 8:00-2:00). Maui May also have a similar shopping place, so ask. about 4 years ago