Buy base tickets for the first trip. You can upgrade the tickets to hoppers, plus water fun or annual passes at the park up to the point before you use your last day. The cost of the base tickets will be discounted from the price for whatever upgrade you choose.
http://everythingwdisneyworld.com/2010/10/upgrading-your-tickets-to-annual-pass/
While much of the info at that site is true, some of it is misstated.
Just know that (within 14 days of first use) you CAN upgrade a ticket that is partially-used or even "all-used-up," and still get the value of that ticket in the upgrade.
(Often, it will be the full gate value of that ticket, even if you purchased the original ticket at a discount.)
If the original ticket was purchased as part of a WDW resort package, it can be upgraded within 14 days of first use, even if the ticket was fully used several days prior (as long as the guest is still at WDW.)
If the original ticket was NOT purchased as part of a WDW resort package, it can be upgraded AS LATE AS ON THE SAME DAY as the last asset on the ticket was used. (By close-of-business that same day.)
This is useful to know because, IF a ticket could not be upgraded AFTER it was used at the front gate of a park, guests who originally only bought a one-day ticket -but had so much fun that they wanted to extend their stay- would not be allowed to upgrade their ticket to do so.
