People who make concrete counter tops use a lot of fibreglass fillers to get them fairly thin but if you wanted it truly light weight you’d probably need to make it out of a dense foam and coat it with something that looks like concrete.
Concrete counter top mixes usually use either much smaller, or no aggregate and use more sand. The mixes resemble mortar more than concrete and they are typically a little harder and less forgiving to work with.
My bathroom is a couple mm of microcement over Schluter Kerdi-Board foam, it's fairly strong. I think it can hold a laptop no problem.
> if you wanted it truly light weight
Aircrete with a good sealant because obviously it would be even more porous than regular concrete https://www.youtube.com/watch?v=z4_GxPHwqkA
There are a range of rendering mixes that look pretty much like concrete, or at least mortar, that would do the job. In fact just rendering it entirely (base coat, plastic reinforcing mesh, top coat(s)) should give a concrete-ish look while keeping things fairly light.