Thanks, folks! I've got ideas.
Here's what I'm thinking: This is going to take place in a clearing, where the Dragon in Elf form is going to offer the PCs a business deal. Basicly, asking them to pay her. The setup is, she's making the same offer to their enemies. If the PCs pay her, and the enemies don't, she'll offer a little help. If the enemies pay her and the PCs don't, they have another adversery to deal with. If both pay, she stays out.
I figure that the following could scare them, but not immediately tip them off:
1) When they arrive at the clearing, the birds are singing in the trees. Then go silent. And when the elf walks into the clearing, they flee from the trees.
2) As she's casually talking to them, she's going to pick up a rock. Play with it. And, as she's talking, idly toss chunks that have simply cracked off, since she's been squeezing on it.
3) The Frightening Presence could easily be a way to keep cocky or overconfident characters on their toes. They get upidy, she pulls a Gandalf on them, as pointed out by Macbrea.
4) When she walks away, her shadow could be...not that of an elf.
The membrain (like the second eyelid from MiB), serpentine tongue, or pupil could work for later meetings.
If the PCs want proof of what she can do, the Dragon made elf would simply ask them to return in an hour. Since they want proof of what she, or her resources can do, she asks them to return in an hour. If they do, they'd find several trees have been broken, shattered, or are in pieces. And the smiling elf is in the center of the mess.
How does that sound, folks? And, if anyone's interested, she's an Adult Green Dragon. Modified, that is.