Greetings, Stephanie!
Welcome to the Disney Parks Moms Panel. I can't wait for you and your husband to visit the Happiest Place on Earth for your anniversary. I definitely suggest stopping by City Hall in Disneyland Park when you arrive so you can grab complimentary Celebration Buttons letting everyone know you are there for this special occasion.
Personally, I would take any amount of time I could get at the Disneyland Resort! For me, just the feeling I get when I walk down Main Street, U.S.A. lets me leave my worries behind while putting a smile on my face. You are more than welcome to visit for just half of a day, but I feel you may love it so much you will want a full day with so much to see and experience.
My number one piece of advice for a shorter day is to arrive early when crowds are the lightest. I would suggest getting in the entrance line at least 30 minutes before official park opening. I like to grab a coffee and pastry from Starbucks in the Downtown Disney District to have while I wait. Once you enter the gates I highly recommend purchasing
Disney MaxPass on the Disneyland app if you haven't already with your ticket. Once your ticket is linked to the app, select "Get FASTPASS with Disney MaxPass" on the dashboard page. This will allow you to make digital Disney FASTPASS selections right from your mobile device saving you so much time while almost eliminating wait times for select attractions. This will truly maximize your visit. With this feature you also get free Disney PhotoPass downloads so you can capture your anniversary memories.
I also recommend spending some time before your trip familiarizing yourself with the park maps on the Disneyland app. This will save you time since you'll have a feel for locations of attractions, dining, and entertainment.
I hope this helps, and have a Zip-a-Dee-Doo-Dah day!
-Jennifer