Re: Hawaiin theme bridal shower..
I know you want simple but I gues, I went overboard with my suggestions as I loved your idea. Let me calm down a bit and I’ll think on simple lines. LOL!
An inspiration board:

Encourage your guests to come dressed for the beach complete with grass skirts and brightly patterned shirts. Hint that there’ll be a prize for the “best dressed”. I love these colours and they will work well with your theme:

An invitation telling guests what to wear:

Favors can include shells, brightly colored leis and other island themed items.
Decor:
For the decoration part, use huge posters to create that sweet effect of a sunset on the beach, along with rubber palm trees, tropical fruit baskets, orchids, a couple of hammocks, grass shacks. And paint the words ‘Aloha’ on some old surfboard and put it at the door for added effect. Use stand up card board palm trees, Tiki torches and island grass garland and partitions.


FOOD:
Trays of fresh fruits and veggies are a must. Hollow out a watermelon and fill with chunks of papaya, pineapple and other fruit. For a visual treat, carve a design into a fresh watermelon. The red design against the green rind will be a beautiful centerpiece. Make sure to get an extra melon so guests can enjoy the fresh summer treat.

Set up a bar with brightly colored glasses so your guests can make their own “Hawaiian Sunrise” drinks!
Another twist is a fruit fondue with steaming chcolate sauce and cute, themed toothpicks.
You can also include full Hawaiian bar-be-que with ribs, chicken and pineapple chunks served with fresh rolls to counteract the sweets.
Another option is grilled mahi-mahi (fish) with mango salsa and some grilled veggies over rice.


For dessert, offer tropical sorbets or fruit kabobs drizzled with chcolate or strawberry sauce.


Have a cake made in the shape of a palm tree, hula dancer, or pink flamingo. Just make sure that it has your bride-to-be’s favorite flavors. Strawberry and lemon are delicious and sometimes overlooked in favor of vanilla or chocolate. You might also consider blueberry, coconut or orange with a complimentary icing.

Real life Hawaiian themed bridal showers for inspiration:
Bridal Shower - pinky0121’s biography - Project Wedding
Picasa Web Albums - Kirsten and Jonathan - Bridal Shower#
Amy’s Hawaiian Theme Bridal Shower, San Antonio,Texas 10/15/05 - a set on Flickr