My personal favorite environment to play paintball without trees is on rocky terrain. While in college there was a local field that I enjoyed playing at that had some changes in elevation along with a gullies, creek beds and a large berm cutting across the field. In addition, there were bushes, grass and a few patches of scrub oak. The trees (the scrub oak) were, at the largest, six inches wide and most were only a couple inches thick, meaning that they were more like large bushes than trees that you typically think of with woodsball. Having trees on the field was incidental as the action of the game focused around the natural and man-made barriers. I remember numerous games where I crawled up over the edge of the creek bed just waiting to see what there was to see. Sometimes it was nothing, sometimes it was an unsuspecting opponent, and, once or twice, it was someone waiting to shoot at me.
This terrain-based field is also fun because it requires a different form of strategy. While you can hide relatively easily behind rocks and in depressions, if someone has the guts to get up on the highest point of the field, there isn't much you can hide from. This would allow adventurous players to locate the few, high places on the field and choose an elevated vantage point to survey the field. Of course, if you identified the opponent at the other side of the field, you still had to work your way across the two or three hundred yards of intermediate terrain to engage. It did, though, add a fun variation to our games.
A second type of paintball that doesn't require trees in one in a field of grass. Grass? Yes, that is correct. While grass would be a lousy location for hiding if you were hiding from real guns, it does a wonderful job at breaking paintballs. While moving through waist-high grass you have the ability to drop down at any time and there is an almost impervious wall around you at all sides. You will get covered with splatter as the grass does a lousy job at stopping the spray, but it is very effective at stopping the paintballs from hitting and eliminating you. The game becomes very fun as you must alternate between crawling, moving and hiding as you try to guess where your opponent will appear.
A challenge with playing in grass is that such fields are not very common. The do exist, often, within other terrains such as meadows in woods or near larger trees. The combination of having some trees and some grass provides a great combination of woods and woods-free play.
One disadvantage, though, with playing in grass is that it a full day of play is not likely to happen. As you trample down the grass and army crawl towards or away from your opponent, shooting lanes are opened and, before long, there are large tracts of the field that have no adequate cover. About the only thing you can do in this situation is to play until the field is "used up" for the day and then return to play another day. A week away and the field will likely look much like it was before you ever played.