I think it's cheaper to get extra days added to the ticket than to purchase hopper?
Disney has that figured out also - so they made it so you
have to purchase the Park Hopper option @ $45 to go to two different parks per day (unless you have individual tickets given to you like the lucky previous poster)

Your ticket will not work at a second park the same day without the Park Hopper added...
Here's how the DIS ticket expert Cheshire Figment explains it on his "Everything About WDW Tickets" sticky toward the top of the page:
You are allowed to re-enter the same park if you leave it on one day, but it can only be used at the one park.
If you put the ticket in the turnstile of another park the display on the Guest side of the turnstile will be "ACCESS DENIED". The top line of the display on the Greeter side will give information about the ticket, and the second line of the display will say "CROSSOVER NOT PERMITTED". The turnstile will not unlock.
There is no combination of override codes that a Greeter can put into the turnstiles which will allow a Base ticket to be used in more than one park per day.
Park Hopping is a $45.00 option which can be added to any base ticket, whether adult or child, whether one day or ten days.
The PH option allows you to go to two, three, or even all four of the major parks per day. Just as a base ticket allows entry to one major park per day, as well as unlimited exits and re-entries to the same park without extra charge, there is no charge to your entitlements when you enter or reenter the second or subsequent major theme park on the same day.