However, most people love dressing up for Purim, and I do like seeing other people dressed up.
Perhaps you've never experience a frum Purim before and don't know what to wear. Purim sounds like Halloween, but are your sexy nun and sexy ghost costumes from 2 years ago still appropriate? Shockingly, no.
Coming from a secular American background, I can't help but compare Purim to Halloween. When planning a costume, imagine if Halloween were truly family-friendly and about having relatively wholesome fun. Costumes will often be nerdy or downright dorky. I'm looking at you, group of people dressed as the Periodic Table of the Elements.
Some costume parameters to consider: tznius (modest), preferably not intended to be scary (too Halloweendik in many communities), and cross-dressing is allowed. Couple/family costumes are common, and group costumes are more common than in the non-Jewish world. You will probably see many "Jewish joke" costumes, playing on Biblical/Talmudic quotes or community stereotypes.
And surprisingly, don't worry about whether your costume is shatnes, a mixture of wool and linen which is normally prohibited by halacha. Apparently Purim is the one day when you can wear shatnes d'rabannan, according to the Rema. If that interests you, you should ask your rabbi what that means.
Here are some costumes ideas for Purim:
Bonnie and Clyde
the Doctor Who reincarnation of your choice
Harry Potter character
the Hebrew Hammer
(Pregnant nuns should be expected in most communities, but I personally don't recommend mocking another faith)
Orbit Gum Lady
Pop culture character
I hope this got your creative juices flowing, and I look forward to seeing all your great costumes!
Next year, remember to hit the Halloween sales for wigs, colored hair spray, costume make-up, and costumes!