After you have been using your laptop for more than a few weeks, it will probably begin to accumulate dirt and crumbs in the keyboard and a light haze over the screen. It's not your fault; you probably handle the computer extremely carefully, but we all live in a world with lots of environmental crud all around us.
You could wait to clean the computer until you return home, but a few simple cleaning supplies don't add much weight to your computer bag. A small artist's brush, a soft cotton cloth, and maybe a small plastic container of all-purpose cleaner are enough to remove dirt and spills from the keys and the case. To clean the screen, wipe it with a soft lint-free cloth lightly moistened with water or isopropyl alcohol.
As an alternative, look for cleaning sheets in sealed packets, specifically designed for LCD screens, such as No. CL360 Notebook Screen Cleaning Wipes from 3M, or Surface Guardian® Wet/Dry Cleaning Wipes from Kensington.
Don't use ethyl alcohol, acetone or other solvents, or any glass cleaner that contains ammonia on your laptop's case or video display; these chemicals can permanently damage the surface of the screen and the material that makes up the outer case.