Disneyland was running on a reservation system that required all guests, including those staying at on-site hotels, to buy their theme-park tickets in advance.
Disneyland Resort is home to two theme parks: Disneyland Park and Disney California Adventure Park. On my visit, I was able to snag a reservation for Disneyland Park, and I was free to park-hop to California Adventure after 1 p.m.
Because I'm an annual passholder, park-hopping was included in my ticket. But for standard tickets, it's an extra $60.
Based on the reservation limitations, I wouldn't suggest paying to park-hop if you're only going for one day.