Hello, Jackie! Welcome to planDisney and thanks for your question!
I'm happy to know that you are planning a trip to the Disneyland Resort! When it comes to
Park Hopper tickets, the cost is per ticket, not per day. So for one flat fee, you could buy a two-day Park Hopper ticket and park hop both days of your trip. If you truly only wanted to park hop on your second day, you would have to purchase a one-day, one park per day ticket and, in a separate transaction, buy an additional one-day Park Hopper ticket (and then make two separate
theme park reservations). This is almost always more expensive than purchasing a two-day Park Hopper ticket.
The other option would be to buy a two-day, one park per day ticket and then upgrade to a Park Hopper at a ticket booth on the second day. But again, it would be the same price to add it only to the second day as it is to add it to both days. And since the parks are so close together, you may find that you want to hop on both days anyway.
I hope this information is helpful. Have a wonderful time at the Happiest Place on Earth!
-Linda