I want to delay visible light (~450nm-600nm) by 10ns. One way would just be to have it travel about 10ft since the speed of light is about 1ft/ns. Could I reduce that length by sending it through some high index of refraction material? Dispersion is a bit of a concern in that case I guess. Any recommendations for what that material should be?
This may not be a complete solution, but you can use mirrors! It's not hard to design a mirror-based optical system which will have the property you are looking for. This reminds me of an interesting Project-Euler question.
Or a less fancy but more practical way would be to use two simple planar mirrors, carefully aligned.
Wikipedia's list of refractive indices doesn't list any above 4, and I doubt you want to pass your light through 2.5 feet of something. You're probably better off just having the light bounce back and forth between some mirrors or passing it through a coiled optical fiber (like Ali and akhmeteli suggested.