Serve up your art mediums in something new, my friends!
Old snack trays are a good choice. Their built in dividers and companion cups for rinse water or other additives make these vintage housewares a wonderful addition to the art studio that can get reused time and time again.
Worn out cookie sheets. Place a wet sponge at one end and your paints/mediums at the other. Work off the cookie sheet itself or line it with waxed paper. A convenient way to mix things, moisten your brushes and it too is a paint palette that can be used over and over again.
Divided storage containers with lids. These little plastic beauties are a better environment for my art supplies than my food, so I like to use them for mixing paints/mediums when I know I'm going to be working on something for an extended period of time. The airtight lids make it a cinch to store things for a few days without drying out.