Usually bachelorette parties are funny informal events where girls just have fun and spend quality time together, like you said, especially if they don't see each other as often as they used to. It's more like a catch up on what they've lost while living their lives independently.
So to answer your question, the right gifts for bachelorette parties are usually funny and original gifts, I wouldn't consider this occasion as one for which there would be any etiquette required.
For example you can have a 100% hand-made painting from the best photo of you and your friend from some time ago, when you used to spend more time together. I guess this would be both sentimental and funny to offer and I say this because this is what I offered to my sister to her bachelorette party last year and she was really thrilled and moreover, also started crying for nostalgia.... Here's a source to use http://www.paintedsouvenirs.com
I wish you the time of your life at the party and remember these events are just for fun....