Well, alo-ho-ho-ha to you, Brian, and welcome to planDisney! Oh, how I would love to run into you in the parks! You are correct that a Santa suit is a no go as costumes may not be worn by Guests ages 14 and older, but let's explore what is allowed.
I read over the
Guest Courtesy and Attire Policies, and I don't see anything you've mentioned that would be a problem. Hawaiian shirts, hats and polos are all allowed. Just as important as attire, are other
Property Rules, such as Prohibited Activities, which include, "Engaging with other Guests or impeding operations while posing as or portraying any character in costume." With that in mind, I would be thoughtful of how you interact with other Guests, particularly children. As always, follow directions given by Cast Members, and have fun in your vacation-mode Santa bound (that's Disney fan speak for dressing in the style of a character without actually being in costume).
Brian, I can't help but think of other activities to continue the theme. Consider having a meal at
Disney's Polynesian Village Resort.
'Ohana is one of my favorites. Be sure to pick up a lei while you're there, which would go great with one of your Hawaiian shirts. Shop for some Christmas decor at
Ye Olde Christmas Shoppe in Magic Kingdom or
Disney's Days of Christmas at Disney Springs. You could even make your way around EPCOT's World Showcase taking photos of all the countries since Santa surely didn't have time to stop for photos on his Christmas Eve world tour. Another fun idea is to commemorate your visit with a
Capture Your Moment photography session with a PhotoPass photographer.
Brian, I hope you have a lot of fun in your festive attire. If you have any other questions, please come back to planDisney and ask us.
Have a magical trip!
Melanie