After Christmas there is often a good supply of usable terrain items.
Pine trees, the bottlebrush type, are in abundance. One thing to remember is scale. A 4 inch tall tree is 400 inches in scale. That's 33 feet, a decent but shorter tree. A six inch tree is 50 feet tall in scale. That is a reasonable height for a tree. A foot tall tree is 100 feet tall. this is still well within the average height of most trees. Dinky trees just don't look right.
Little houses can be useful. Some need repainting and other such, but most work well enough. A good way to determine the scale of a house is to measure the door. 3/4 to 7/8 of an inch is about right.
There is a lot of variety in the things available. The embossed vinyl with brick or stone patterns can be used, but be careful of the size of the pattern.
Of course, as with everything, there are no terrain police. Cool is the fuel. have fun.
Always have a penny and nickel with you - the penny is 18mm tall, the nickel 22mm tall.This allows you to measure all sorts of things. Those many on-sale ceramic houses will do great with a little repaint job - check with the penny for the those door ways for the right scale. And those 'little' trees could be part of new growth in a forest of pines Don't forget to check the toy stores for tubes of animals, plants, and other items. This one company (fails me atm) makes long tubes filled with animals and trees, etc. as educational toys that are correctly size for AQMF. ps: I've got a whole box of various size pine trees and several rows of various buildings in my game basement that I purchased several Xmas seasons ago.