I've started collecting a range of non-scale specific 'flames of war' battlefield-in-a-box terrain pieces. They were designed for a world war ii game called flames of war, which I think is 15mm miniatures, but if you just use the terrain scenery pieces (not the buildings and stuff) they work with any scale just fine!
So far I've got the river box, which has lengths of river that tile together to go horizontally or vertically across a table. It also comes with two bridges (one stone, one wood), and some of the river sections have 'ford' crossing areas.
Then there are hill sets which can be flocked if desired. I left them raw rock, so they go with grass, sand or snow just fine. This photo has the small hills set, which is two small pre-painted rocky hills.
Then there are the pine woods sets. These per flocked/painted pine tree bases were from the small pine woods box set. I have two of those boxes, for a total of 12 tree bases. (The smaller individual trees are model railroad trees I had years ago.)
The grass mat is the old games workshop gaming mat. I don't think they sell those anymore, I came across one on a shelf in a hobby store, and it was still in perfect condition :)
They Battlefield In A Box sets have some other stuff too. There's a large pine woods box (which I don't have) that has much taller trees in it, but I like these ones, because they fit in with my 10 and 15 mm miniature games as well as things like battletech. Anyway, here's a photo showing some of these pre-painted terrain features, that come ready to just stick on the table and play!