Creative Licensing

I've played a good number of role-playing games, and most editions of D&D, and I really like how 4th Edition Dungeons & Dragons goes a long way in making sure that players have a lot of options on their plate. Most of these options are determined by race and class, though there is a short-list of things that every can at least make an attempt for (grab, bull rush, and melee/ranged basic attacks, for example).
I enjoy this design move because it provides a lot of thematic and effective decisions for players, so anyone playing the game has an understanding of what it is they can do, or what they should be doing. This is great for new players, or even veterans playing a new class.

What about "non-standard" maneuvers, however? Stuff like swinging from a chandelier, knocking over a book case, or dropping a tapestry on the bad guys. Most of my players tend to overlook the environment and just use what they know works, and honestly I think this is a syndrome of playing in older D&D editions where in most cases it was a bad idea to try something off the cuff. You never tried to make a grapple in 3rd Edition because it was very hard to pull off, and when you got it to work you wondered why you'd bothered since it didnt really seem to do much.
The same thing could be said for many non-standard actions, which probably required several very difficult rolls (if a roll is even called for) and amounted to some damage...you got better and more reliable results with your standard array of swords and spells.

When I design encounters, I try to put in some interesting environment elements that the players or monsters can work with. I then decide what I should immediately inform them about, or use their "passive" skills to determine what they should know. I might point out a book case, for example, and tell them that it can be knocked over. I might just mention it and wait until they try to use it...but I might also use it on them. For things like fungus, or glowing crytals, I rely on the passive skill. If its high enough, I'll tell them what it does. If not, they need to call for a roll to see if they can puzzle it out.

Case in point, in the first Songs of Erui adventure, there is a chamber with a massive glowing crystal set in the floor. Grynn was able to determine that it could be used to channel cold magic, assuming one could harness it and direct it. When they got attack by undead, some of the casters started doing just that, and proceeded to ransack the chamber with ice blasts. Once Grey and Grynn got them away from the crystal, Grynn took over and started using it against the bad guys.
Of course, some players like to throw things at you that you werent expecting. I've said it before: Josh is one of the players I can count on to always try something really bizarre or different, about once per game (if not more). He pitches me an idea, and I give him the odds. Sometimes its easy to figure out a mechanic, but sometimes not so much. It doesnt always work, but thats the fun of risk versus reward.
For example, how about a flying drop kick? Gimme an Acrobatics or Athletics, and Strength vs. AC roll, and you'll deal 1d4 + Str mod damage. If you succeed on the skill roll, you'll also knock the guy prone (and if you succeed very well, you wont fall prone). All in all, it adds another die roll to the attack routine, and doesnt give him an awesome attack that is better than his usual fare: its effective and fun.

Mostly, I have a hard time wanting to allow non-standard actions that dont rely on the environment, because I'm concerned that some players might try to do it all the time. I actually give them a clause in many situations, "I'll allow it for now, but I reserve the right to prevent you from doing so in the future. If you're okay with that, lets continue."
Its easier to allow Grynn to channel powerful beams of cold energy when he has to make Arcana checks and touch a massive crystal set in a chamber. If he could use Arcana all the time to do something like that, well...its basically like giving out another at-will attack.

One thing I'll make clear in case I havent before: I'm not a fan of arbitrary decisions and resolutions. I'm not going to let any character give it a shot, and just say it works, say it doesnt, or even give them the same odds. You want to push over a book case onto some monsters? The strong characters are going to have a better shot at it than, say, most wizards. Similarly, if you want to swing from a chandelier then you'd better be pretty damned acrobatic, otherwise you're going to fail.
I'm more than willing to let my players try creative actions that arent covered by the rules. This does not mean it will work, or even be as easy. It might require an extra roll to see if we can even get to the next step, or to avoid having something bad happen to the character.
You wanna use thorn whip to pull a fellow team member out of a bad spot? They're going to take damage. You want to use thorn whip to create a rope for a fellow team mate to climb out of a pit? Gimme a Nature check and we'll use that in place of an Athletics check to Aid Another. If you roll bad, they'll take damage (or perhaps even fall from grabbing thorny parts of the whip).
Of course, if druids start abusing thorn whip for all manner of bizarre instances, then I wont let them do it anymore.

I think the best way to encourage your players to do things other than whats covered by powers is to start out by making such extra options very clear for them. If they decide to give it a shot, make sure they know what check(s) will be used, and at least what general effect it will have. For best results, try to make it stuff that will give what they can already do an extra oomph, or so that the damage is roughly on par/adds a condition that they cannot easily do.
A falling book case, for example, might deal "okay" damage, but knock the target prone and immobilize them for one turn. Thats something that most Strength-based characters will want to use, since they cannot easily do those effects with an at-will attack. Hell, you might say that the books grant a one-time attack bonus with a Fire effect and/or bonus fire damage.