You can add a flash of light at the end of a tunnel using the env_lightglow entity. They one is pretty easy to use.
Getting nice lighting for outdoor maps requires a lot of tweaking, unless you're a light expert.
Just add a light_environment entity, and set brightness similar to the color of the sun and ambient similar to the color of the sky. The fourth number in the brightness input box is the actual brightness level. Depending on what time of day your map is, it can range from 250 to 800, or even more.