Regardless of who's organizing and paying for the party, be aware that planning any event takes a lot of time, effort and money.
Given the catering, invitations, booze and everything else, an engagement party adds up! If the hostess asks for your menu or beverage requests, offer preferences, but leave the final call up to her.
Unless you're determined to spare no expense and have the best of everything (see 10 Signs You're Turning into Bridezilla), remember that this is just the first of several soirées being thrown in your honor. Have fun, but save the caviar and Cristal for the reception.
One last word: Don't arrive empty-handed to the party. Bring a hostess gift or, at the very least, a thoughtfully written thank-you note. If you leave your hostess with a good impression, who knows? She may step up to throw your bridal shower, too!