I purchased density blocks from Ward's a few years ago. They are excellent for understanding density. From just handling them for the littlest of students, to measuring them to compute volume for older students, to weighing them and computing density, or in conjunction with a spring scale to explore buoyancy.
I like this set of blocks a lot. These blocks hold volume constant so the difference in weight is obvious. The cube shape allows the volume to be computed easily by multiplication. There are a number of different materials. The floating blocks have enough variation in density that there is a block that is almost fully submerged and another that floats almost completely out of the water. As a warning, dry the metal blocks off well to reduce oxidation. A thin coating of mineral oil (or other non-food oil) will help preserve them.
There are equal volume rods available from Home Science Tools: http://bit.ly/fe5seJ While I normally like that site for their supplies and prices, this density set is not nearly as nice as the blocks. The materials are very limited (only four compared to 12). The cylinder shape requires using the formula for computing the area of a circle before the volume can be calculated. Though it could be as simple as multiplying, the area of a circle is not usually taught, proven, and understood until algebra. Though the volume can be measured by submersion, I appreciate the more accessible and straightforward cubes.
Ward's e-mail subscribers can get 15% off a purchase for a limited time: http://bit.ly/hz3LR0
From Edmund's (better price--but check shipping): http://bit.ly/h0rovC
As a corollary to the density blocks, this set of equal-mass density rods can really help cement the density concept. Each of the rods is made of a different material and the mass is kept constant. To get the same mass from materials that vary in density, the volume needs to change. The brass rod is only about 1/2 an inch long. This set is nice, but I find it no replacement for the density blocks. http://bit.ly/gdDJhj
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