Pg 42 of the 4e DMG gives guidelines for what one-off stunts can do in terms of damage and special effects. The 4e power system can definitely feel like a limitation for some people – “I can only do these things printed on my sheet” – but this is a false perception. I think encouraging stunting is a twofold process:

1) prepare stunt-friendly situations and scenery. Chandeliers, ropes, walkways, high tapestries for swashbuckling action, braziers and cauldrons of bubbling liquid for imprompo area attacks etc. Think about how you are going to describe it beforehand as over-the-top descriptions are one of the funnest things in stunting.

2) Inform players (and the DM) that stunts are neither automatically suboptimal nor auto win buttons. Let players have fun with improvised stunts without making them either mandatory or complete wastes of time. Failing in a stunt should be embarassing at worst, except in the most exceptional of circumstances.

I do find that players most often resort to stunts in bar-room brawls, on pirate ships, or in unusual circumstances when there are prisoners to save, a maguffin to snatch, or a ritual to prevent. Especially if they are feeling stretched and desperate.