One is Piaget's capacity idea - you know - short fat holds the same as long thin. Use glasses of different widths/heights and coloured water.Get the kids to estimate before you pour. (Young kids often assume the tall will hold more - it is so funny when they stare in amazement as you keep pouring from tall to short and it doesn't overflow!) But older kids can do things like use measuring cups and pens to mark things out.
Another idea is to have rocks, pebbles sand and water. Get a large glass container and fill it with rocks. Then add pebbles, and they fit (around the rocks - give it a shake)! Then add sand, and it fits! Then add water and it fits! You can show how this works by emptying then filling with water, then show the water spilling out when you add rocks, pebbles, sand.