If you do a sandwich core with a lightweight material, think very carefully about whether you need a core closeout to join the top and bottom skins together in shear. If you don't, and the core material is soft, you can end up with a flexible bench top or a shear bond failure where the materials meet (or both).
Luckily, for something as simple as a bench top, a core closeout can be a strip of MDF around the edge (or whatever the top/bottom skins are made of) that is the same thickness as the core material, kind of like the rails and stiles of a door. Small weight penalty, but massive increase in stiffness by just preventing the top and bottom skins from moving relative to each other in tension/compression while loaded in bending.
This may not be strictly necessary if the bench top is mounted in a table frame, but rather necessary if the bench top is laid on top of sawhorses like you had mentioned.
The core and the bottom skin could be made of lots of things, like a bottom skin made of thin aluminum sheet to gain some extra core thickness and save more weight. Anything capable of being easily cut with a CNC router and with enough strength to be loaded in tension (as the bottom skin) could work... I'm assuming the top skin will be out of 1/4 MDF or something as the normal sacrificial layer.