Terrain Options Try mixing 2 situational principals of physical effect consequences (Item effects/Ground type. & Body Movement changes) and then, later, subvert it with applied traditional canonical spells.
Terrain bonus Reward the players. Build some Useful Personal - Terrain Tables and Large Item uses - supply's (traps, picket lines, lrg. rock moving/levering tools, hiding & covers/blinds) to Enhance standard terrain features for combat.
OverLand Travel Supporting Terrain perks means Getting to them.
Encourage Doing or Hiring Scout-ings, by adding favorable features based on Scouting Point(s) rolls. Plan Terrain potentials or hazards by Terrain knowledge, Scout Path planning and choosing safe wagon ditch & reclaim location.
Survival fall-backs if mission is fail AND Terrain Escape Plans, Be-it Arching from a humble tree (rope swing escapes, beware of height to rope to length-to-ground curve) or Getting off that ledge if their party has to fall back. Supple Tree's bent towards short cliff and tied -STAKED- down 'Sproing!, using a small pully-wheel @top, Pitons 1/2 way down and rope upto pulley (optional cutdown tree w/ rope stakes for overhang) then droped down rope to the waiting "cliff floor walled" pirate/Scout. Archery & boulders. Foolish players.
In under ground/Stone Rooms settings, Be Creative - Offer players store made adjustable stick frames and paper/cloth coated rockchips and paint texture Ambush blinds. Offer those stalagmite/stalactite mounds or non visibly obstructing alcove rock indents some rear ambush potential. Foes typically charge in, walls are never even or truly clean cut, corners exist. Never underestimate a pouch full of Rock Dust b4 knockbacks.
Slime & oil flasks, Bear-traps, glue-muck plates!light & massively encumbering!, spike strips rock paint/dirt (or magic) camouflaged, and 'Smirk' Pole-Arm side pushes and stumbling group bumbling's tripping over their own feet and their friends to stop. Figure out Who's in the party is the "Fast Runner" designate.
Reward town-skills, Painting, signal use, lumberjacks wood/stone-working, Taxidermy(preserved, stuffed! animals make clever step-on gas/irritation-powder and triggers- kick-the-can) Sailor (wavy floors) ect, Used or earned through Terrain modification use.
- Magnetism & metal Vs Levitation spell effects.
- Open battles in principal. Reward Scouting With Terrain Defense Situations and suggest "Goblin Lure" Tricks. Campfire-smoke signals, Drums, Screamer whistle arrows. Terrain uses, Forests: Heavy oaktree limbs under-notched wedge and rope over tree nail-Large Rock on small rock on a slip-knot roped braced Tripod, arrowpannel or rope pull trigger, ect. Instant Cover!! Real Random Encounters in the Open are about bringing the Enemy before you, Not INTO you unaware!
- Gravity lift (weight changes or per metal encumb mass) that alternate Str & Dex numbers by floor square or wall/pillar objects (falling down, Will/Int rolls each action) VS Movement By Grapple. Plant's Grappled/released Swinging. Example, Boating yourself forward with a Pole Arm to the next grapple "swing set bar-Rung" by the plants grab.
- Waterbed style "Weight lifting Floors" Wave-Rolling settling Stone/Metal/Magic. Partial cover by floor lifts, lighter allies Jump trampolining on the wave-barrier to push down and move Armored heavy's forward. Ledge/outcropping arching being massive hit/miss advantages. Creative combat modifier potentials and leap-frog marching attacks by outnumbering foes. & the favorite 'Throwing the Half-ling Wink
The rotating Statue cone-ial magic attacker has many an interesting effects in reverse.
Think magic energy flows + metal covered floor conduction & reversed room magnetism.
Resisting Downward swinging and falling "metal weapons items". Anything triggering levitation spells is going to inspire creative options.
Area Push/Pull Logic's Throw/Fling weapons getting a "bounce deflect" upto their STR bonus in momentum before sticking to a wall. For Continued, per turn fun, put a Deflect Missiles spell pillar in the room with a set STR throw deflect at waist high level boomeranging.
Optional gimmicks Levitation stability, magnet angling + metal sled/shield riders/lancers.
Cover/Obstructions 15 to 20 foot high+ rooms and zig zag pathed floor tunnel hallways, using 5" to 6" pillars metal (steel core copper coiled, wood tubed and art etched softmetal plated) Push/Pull + & - magnetic charged, partial cover and side Curving arrows/throw weapons. That Orc with a Mirrored Glave-blade? He C 'U he Bola, boomerang-blade, and poiky metal sharpie handful bit's to you hurt!
Weak foes and advanced tactics Skilled environment humanoid defenders. Wood/stone weapon advantages (by metal deflection disadvantages) strait firing Arrows, oiled/wet floors & hovering by shin/knee Sliding kick-offs, dodge mobility perks. Tunnel drop in entrance ambushes with magnetic knee-guards.
Planning vs Design TRAVEL & Outdoors. Many DM Game managers see item-Carts, Horses/ox's, and hired NPC mercenary's - Guards/Retainers as cumbersome, Game bogging & paperwork or Force Size brakers and Fealty & Leadership nags. But consider the threats & times, Monsters roam the hills, Humanoids ride beasts and Sane Adventures crawling into caves Need a defensible fall-back camp (Cart sized Barrels for, tarpit seal, flame oil traps, Tools, Labor to picket & fortify) And a few rider's to towns or a relay chain for getting help. (out classed and swarmed/hiding)
TAKE Away Their Weapons? NO! When in Doubt for Player Supported mobility Awareness and gaining the players terrain observational Interests, TAKE THEIR BOOTS!
'Nothing slows mindless, healing addicted, Int6-less rush-in players then a set HP dmg limit for foot arch's, with ratio-scaled neg mobility mods.