Here's one I used last night:
57 or whatever we're at) The PCs are on a giant slab of ice/rock that breaks off and begins sliding down a mountain side/glacier. It tilts from side to side, forward and back as it goes down the mountain. Creatures must make Acrobatics or Athletics checks at the starts of their turns or fall prone. At the end of each creature's turn, they slide 1 or 2 squares towards which-ever edge the slab is currently leaning towards.
Note: In my game, everyone was instead inside a giant treaded vehicle (imagine a fantasy semi-steampunk Jawa sand crawler) traveling down a mountain road. Two of the characters were inside being transported to a Hegemony (evil empire) base and were busted out of their cells by a Resistance agent just as the other two characters assaulted the top of the crawler by airship. The agent's sabotage destroyed the breaks, sending the vehicle careening down the road as the PCs fought their way through the Hegemony troopers and elite Dragoons to escape (by leaping back onto the airship as the crawler careened off the mountain side). The slab of rock/ice is a slightly less campaign specific version...
58) Another one I've used: The party is fighting on a frozen lake/river. Fire spells/auras/zones/attacks melt the ice in any square they target, each round of fire weakening it more.
Round 1: Water covers the ice. Anyone crossing it must make an Acrobatics check or fall prone.
Round 2: The top couple feet of ice melts. It is now difficult terrain.
Round 3: The top square melts. Creatures must now swim in that square and any creature that does so becomes slowed(save ends) unless they have cold resistance/immunity.
Note: In my game, they fought a skirmisher with a fire aura on a frozen river with shattered trees and other debris clogging the shores beside the river, so there were melty and melted bits all over the place that weren't there at the start of the battle.
Battlefield chaos: If the party is fighting on a larger battlefield, random effects happen.
59) Rain of arrows: "Off-screen" archers of either side unleash a flight of arrows that target a group of their enemies on the battlefield (though friendlies might be caught in the attack as well!)
60) Stray trebuchet: A massive boulder (1x1x1, 2x2x2, 3x3x3 or bigger!) crashes down somewhere on the battlefield. At lower levels, it lands somewhere where it doesn't hit anyone, at higher, it might make an attack roll against them for massive damage, prwning, and/or restrainment. If it misses, they shift out of the way of it.
61) Greek fire/magical flames: A blast of fire strikes the battle, lighting things on fire in a burst 1-2 and possibly spreading.
62) Loose horses/worgs/elephants/other mounts run through the battlefield, trampling those in their path. Creatures can make athletics checks to snag one as they go by and so have a mount for the rest of the encounter (and maybe longer!)
63) Enemies with banners/sharpened stakes: An enemy leader has a banner that they plant, functioning like the player ones (granting bonuses to X to any enemy within 5 of them). Other enemies might have stakes that they plant in the ground, dealing damage to anything that moves through them from a certain direction.
64) Crumbling wall: The wall/structure the encounter takes place on is crumbling. Each turn, some part of it breaks off, creatures on them can make Athletics checks to leap to safety, and the squares next to the areas that broke become difficult terrain as they crack(and they will fall off next round).