I don't know if KaBOOM can accept donations and then put them directly towards your playground project. Someone from KaBOOM would have to answer that. My guess would be no, and a suggestion for you to consider would be printing off a picture image of the playground design itself, and then having little prices next to everything... or a printed price sheet... but estimate things that you think people can afford to "buy", i.e. charge $200 for the slide, and $50 for a monkey bar, that type of thing. Call everything by it's given name by the manufacturer (e.g. Twirly slide) or come up with a creative name (e.g. Twisty Turbo slide)... the idea is to show people what their money can "buy" as opposed to asking for just a straight monetary donation. It may be more fun for them to "shop" and then that determines how much they donate. Then they can mail their purchase and money to you or someone in your wedding party to handle the donations.
If the issue is getting a tax donation receipt, then is your playground associated with a city or a school? I would think that either could issue a receipt for donations that they can use on their taxes, in that it is a donation for a public purpose, not to benefit any individual. Our school district has a form letter that they use to provide tax receipts for people, even though technically schools are not 501 3(c) organizations.
In terms of an online donation mechanism for a small project, I've run across something pretty interesting.
http://www.chipin.com/
It'll handle the credit card operations for you through PayPal. PayPal is nice because it's been around forever and has good security. It's what much of the money exchange on EBay is based on. ChipIn gives you a fun little "widget" that you can put on your web page -- any web page, not just KaBOOM! pages -- that makes it easy for people to donate. You just create your accounts (an account on PayPal AND an account on ChipIn) and then ChipIn gives you some HTML code. You copy and paste that code into your web page.
Sounds easy, huh? If you have a KaBOOM! project web site and want to use this, give me a hollar via email. If I hear from you that you are interested in this, I'll write up some formal instructions and put them someplace useful.
Amy