To expand on the food thoughts, what if you did a Hawaiian luau appetizer-style? Skewers of grilled chicken with teriyaki dipping sauce would work and definitely serve steamed rice and macaroni salad and a fruit salad. (Skipping any ideas of Spam here.) The cake could be a pineapple upside down cake and you could serve fruity (nonalcoholic) drinks in fake coconut cups and everyone could get a lei as they enter (let www.orientaltrading.com
be your friend). I'd skip any ideas of coconut bras or grass skirts. Unless said grass skirt is being worn by a table.
Oh! And if you want to play that game about not saying the word "baby" or someone takes your clothespin, use the leis as the clothespins! Whoever has the most leis at the end of the party wins!
Get a couple of yards of Hawaiian shirt fabric and use that for a tablecloth. Make the invitations in the shape of a hibiscus flower or flip flop if she's having a girl, or a surfboard or sea turtle if it's a boy.
OK, I'm out of ideas for now. Maybe try to make the games with a Hawaiian twist like the aforementioned clothespin game. Not sure what else. I'll try again later.