My focus here will be creating encounters that are resource-draining but tactically (and usually narratively) irrelevant. Perhaps irrelevant is too strong a word, but in any case these are encounters that don't pose much of a threat and that are over very quickly. There are several ways to evoke this feeling, the simplest probably being an at-level or lower encounter, designed more or less "normally." The problem with this is that monsters that are too low level are a complete non-threat (PC attacks will virtually auto-hit them, and they'll be unable to hit the defenses of the PCs), and those that are a threat tend to have enough cumulative hit points to drag the encounter out to a decent length. One way to keep combat short is to simply keep the at-level monsters, but use fewer of them. If the PCs outnumber weak monsters 2:1 it shouldn't last more than a couple of rounds, but during that time the PCs should take some damage (which they'll need to spend surges to fix), and might even blow a daily if they misjudge the encounter's difficulty.
Here are a few more ways to create quick and dirty "filler" battles:
- Use terrain that gives the PCs a massive advantage. Describe it so that it's really obvious that the PCs can use it to quickly dispatch monsters. If you want to maintain the level of danger present in a standard encounter but make it happen much more quickly, have the monsters use this advantage first, but allow the PCs to quickly turn it against them. This way the PCs take boatloads of damage in round 1 (or even a surprise round), and then end the fight quickly by subjecting the monsters to the same effects. Some examples include pools of lava or acid that creatures can be pushed into, buckets of lava or acid that can be thrown, large and heavy objects that can be knocked onto creatures, or containers of explosive substances that can be ignited to create a nasty explosion.
- Use cowardly monsters. After a couple of rounds, perhaps even before most of the monsters are bloodied, have them flee. Maybe they'll lure the PCs into a trap or ambush, or perhaps they'll simply reinforce a later encounter. Depending on the situation they might just run away, and sometimes the PCs may simply let them. If the PCs insist on chasing them down, instead of rolling loads of dice to whittle away at their HP simply describe the situation narratively: "knowing they're outgunned, the bandits use double moves to run full speed, and the party finishes them off with arrows, javelins, and spells." It might be more interesting, however, to use creatures that can actually flee effectively (flight, fast speeds, invisibility, phasing, burrow speeds).
- Finally, use copious amounts of minions! These guys were practically built for encounters like this. You'll definitely need to tailor encounters to your party's capabilities, though; a party with strong AoE capabilities might be able to kill a dozen or more minions in 1 round, whereas parties lacking AoE would end up being in for a really long battle, which is the opposite of what you're going for! Also keep in mind that if there's a disparity within the group, as there almost certainly will be, AoE-capable PCs will be the stars of the show for a while, whereas high-damage single-target strikers might feel quite useless. Wizards with Enlarge Spell can be particularly problematic, as the areas they're capable of affecting can be quite large.