Wooden boxes, lots of wooden boxes. Minimum of about a foot per side, a couple maybe 3 feet by 2 feet by 10 inches high. Maybe some 4 foot cubes if you have available space (different sizes should nest together), use 'em for tables. Great for storing lots of small stuff. Use as props either painted white or ruggedly unfinished, throw some fabric over them and create a multitude of shapes and arrangements for sets. The one I use most is about 16 inches square and 20 inches high. The "official name" if you wanna buy'em is apple crates and they run $35 up... get some 1/2 to 3/4 inch plywood and make your own much cheaper.
Stools and small chairs. Thrift store or yard sales. Sit on em, use 'em for props, have a variety at hand. Don't pass up a beat-up looking one. A can of red spray paint can do miracles.