Our upcoming product, Terrain Toolbox features dozens of sample terrains, along with advice on how to use them. Here is a preview of one linked pair that you can find in the book.
Highly Flammable Oil (Sample Level 2)
The floor is slick with oil, and sharp smelling fumes rise from the liquid.
Skills: Easy (DC 9) Nature or Dungeoneering to recognize the danger of falling. Moderate (DC 13) Nature or Dungeoneering to realize its flammable nature.
Set Up: Begin with any number of squares of highly flammable oil. Squares should be clustered in puddles of at least 4 adjacent squares.
Effect: A creature that enters this oil must either move at half speed or make a moderate (DC 13) Acrobatics check to move at full speed. If he fails this check, the creature falls prone.
Trigger: If an area or close attack that deals fire damage includes oil squares, or a creature taking ongoing fire damage ends its square in an oil square, the highly flammable oil become flaming oil.
Flaming Oil (Sample Level 2)
The floor bursts into fire.
Skills: Automatic for damage. Moderate (DC 13) Nature or Dungeoneering to know how to extinguish the fire.
Effect: A creature that begins its turn in or willingly enters a square of flaming oil suffers 5/tier fire damage. At the end of the round any adjacent square of highly flammable oil becomes flaming oil.
Trigger: A close or area attack that deals cold damage turns flaming oil into highly flammable oil until the oil is ignited again.
Encounter Building: Because highly flammable oil reacts to fire, its true potential only comes out when either the PCs or their opponents can use powers that deal fire damage. If you want the flaming oil to primarily favor the enemies, they should also have fire resistance or immunity.
As noted in the exploding keg entry, the two can be paired effectively for an explosive battle.
Variations: Creative players might be able to find other ways to extinguish the flaming oil, or you might allow them to use certain skills. Note that it real life, dumping water on oil fires actually spreads them.