I've had men come up to me at random in public situation to tell me I look nice, and here's three things I'd suggest to make sure you come off like one of the non-creepy ones:

1. Say: "Hey, I'm not trying to hit on you or anything, but I just had to tell you --" or the equivalent.

2. Compliment something specific about her outfit or hairstyle instead of just saying you think she's pretty or you like her smile. This a) tells her you're complimenting something she *chose*, and not just staring at her female body, and b) complimenting her specifically & not just random women. But keep it short & sweet; more than a sentence is too much.

3. Don't hang around once you've said it. If it's in a situation where you'll have to interact for awhile, say it as you or she leaves; if it's in passing in the street or a shop, move away afterward; if it's in a bus or waiting room or something, have a book or ipod to turn your attention to instead. If she wants to continue the conversation she will, but don't give the impression you expect her to or will react badly if she doesn't. (And just standing or sitting there next to her can give that impression. Even if you think you're a completely nonthreatening person.)

The card thing would come off as creepy to me no matter what it said, tbh, because it implies that you came prepared, which screws with the power balance even more than gender already does. Unless it's like synecdochic suggested and is utterly nonspecific, in which case it would just come off (to me) as trite, but would be okay. (But then, I'm one of those people who got sick & tired very quickly of being told to 'buck up and smile!' No, it ain't my responsibility to look cheerful for you, sir.)