One of the requirements for the liquid base of oilfield drilling fluids is high density. Besides preventing the chips from falling out of suspension, the increased hydrostatic pressure helps prevent the sides of the well from collapsing before a steel casing can be applied.
Oilfield drilling fluids are based on alkali metal formate solutions. Sodium, potassium and even cesium formate solutions are used on a multi-ton scale. Reasons for their success in this application are the high solubility of formates and the low toxicity of the solutions. Sodium and potassium formate are certainly dirt cheap; I am unaware of the price of cesium formate.
According to the relevant document dowloadable at http://www.cabotcorp.com/solutions/products-plus/cesium-formate-brines/formate-technical-manual cesium formate brines of up to 2.50g/cm3 density can be prepared at 15.6C. I would assume solubility increases at higher temperature.
That may not be enough to float regular glass, but should be enough to float borosilicate glass, which according to http://www.udel.edu/chem/GlassShop/PhysicalProperties.htm has a density of 2.23 g/cm3. Similarly, aluminium will not float in this solution but boron will.
Looking at the other answers, I note that Clerici solution uses a combination of formate and malonate, presumably to improve solubility by having a mixture of ions. This strategy should in principle work with cesium. Probably the ultimate non-toxic solution would be a cesium polytungstate solution.
One issue you will find is that materials of similar density tend to have similar refractive index. Transparent glass in a high density transparent solution may be almost invisible. Also, be careful with aluminium in contact with alkaline solutions. If the pH is high enough, they will dissolve the oxide layer, allowing the aluminium underneath to corrode extremely rapidly with evolution of hydrogen.
Edit: Thanks to John Downs for the following info: Cesium Tungstate solution with density of SG 2.80 is available from various sources. This is interesting though it less than the density of the sodium polytungstate solution mentioned in Aabaakawad's answer.