How to make combat encounters interesting

Don't draw every detail.

This came up for me when I was trying to make forest encounters feel different than open counters; I'd draw trees randomly on the map, it took time and just made the encounter a mess. Instead, add static effects to the encounter that says something like...

You are in a forest
Small trees and brush provide some cover and make it easier to hide; the a single tree is too small to provide cover but at a distance, there are enough trees that get in the way to make ranged combat difficult. Anywhere off of the path is considered difficult terrain and grants 1/2 cover on all ranged attacks (3/4 cover after 20 feet and full cover at 60 feet).

I've done similar static effects in other settings, like a swamp causing the poisoned condition due to the smell. The same can be used for a cold or hot area, each round you spend in this setting, take X damage or roll 1d4/1d6 damage (resistance of the appropriate type cancels this effect).