Lots of images to follow.
Spawning of Bob's takes a free approach to terrain construction. If not free then very cheap.
I spent a year picking up bits of litter and nice looking stones. Then I started cutting bits off living plants.
Anyway, after counselling.....
Modular table (just like realm of battle!). 3mm MDF with exactly 12 inch adhesive backed vinyl floor tiles. You can it under everything. green grey slate looks swampy enough to me. It is cut into sections 2x2, 2x1 and 1x1 and has adhesive velcro tabs under so I can make a game table anywhere from 2x2 to 4x6 (and beyond!) No specific photos, but it is under the others.
Hills. Again modular I have 6 quadrants allowing corner, edge and free standing placement. The bases and plateaux are vinyl tile again. I built them to 1 inch height then squirted expanding foam around the edges then carved either broken masonry or just rough surface after it was dry. Cheapo spray paint with me holding a twig in front of the jet to make it splatter (white makes good guano....). Spray adhesive then foam flock and lichen. Hot melt glue on some sprigs of fish tank plants.
If I was doing it again I would make them higher, and not use glue that dissolves polystyrene to try and put it together (expanding foam however, does not dissolve)
Here is Bob himself on a shrine. Carved from a blob of expanding foam (the spires are cut outs glued to the back) spray painted and flocked the the same way. Vines and roots are strings of hotmelt glue, later painted citadel poo brown and with flock and foliage glued on.
WIP progress obstacle / fence. Fossil T-Rex, gap filler (white) brown paints. Still needs a dry brush of bone, flock and foliage.
Swamp / Lake and river are different colour tile base with sand or flock glued to give texture. I put some small fish tank plants on tiny bases for reeds etc. Together the swamp make about 12x12. The river is 4 inches wide and can run 4 feet if needed. Modelled by Kroq-Gar and my scratch built dianoga eye stalk.
This is a temple or hill, depending. It is 3 layers of some sort of foam which had perished and crazed in the sun. Usual spray paint and flock treatment. The chocolate sauce is meant to be vine roots. Another coat of lighter brown required. The two statues are functional. Poor saps without magnetic movement trays can use them to "level up" on the tiers of the hill.
Trees! Forest bases are (bad) flocked tile bases. The original trees were rosemary sprigs, spray painted green after the leave s went brown. Trees stuck down with hot melt glue (painted) roots. After the leaves fell off entirely

they are good for Sylvanian scenery, if anyone wants to make an offer....(Scalenex?)
My later trees are rosemary and lavender stems (woody plants) with glued on plastic leaves. I've got some big enough to eventually make an arcane fulcrum.
Summary: Jungle ruins are easy. Spray adhesive, flock, hot melt vines, fish tank plastic plants. Carving ANYTHING from expanded spray filler foam (whatever you do starts off looking like stegadon poop) is very therapeutic.