Hello, Rachel. Welcome back to planDisney!
Currently, when you purchase a 1-day
theme park ticket to the
Walt Disney World Resort with the
Park Hopper Plus Option, you must visit the theme parks
and water park (or other included experiences) on the same day. However, if you purchase a ticket with more than one day, you typically get a window of dates to use your theme park ticket and your admission to the water park (or other options). For example, if you purchase a 2-day theme park ticket with the Park Hopper Plus Option, you'll have 3 days for which you can use your 2 theme park admissions and your 2 admissions to include experiences. You can typically play around with the dates on the calendar while purchasing your tickets to see how long and when your ticket will be valid. To answer your second question, while you can usually add a day to your 1-day theme park ticket, you should do it before you visit the resort or, if you're planning on doing it during your day in the park, do it before you leave the theme park that day.
Using the Park Hopper Plus Option can be a great way to see and do more when you're visiting. I like to pair up
Magic Kingdom Park and
EPCOT or
Disney's Hollywood Studios and EPCOT. Why? Well, they are easy to hop in between. You can easily use the
Walt Disney World Monorail to hop between Magic Kingdom Park and EPCOT and the
Disney Skyliner to hop between EPCOT and Disney's Hollywood Studios. And while I love the grandness of the main entrance to EPCOT, there's something special about the entrance you use when you enter EPCOT after disembarking the Disney Skyliner. You are at the International Gateway, in the heart of World Showcase, right in between France and the United Kingdom. And, in case you're missing home while on your trip, Canada is pretty close to this entrance as well!
Rachel, enjoy your
summertime trip to the resort. If you have any other questions,
I'm right here!
Some dream it, some do it, some do both,
Matt