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.