HI Aida!
Disney park tickets are needed for any of the 4 Walt Disney World Theme Parks (Magic Kingdom, Epcot, Animal Kingdom, or Hollywood Studios) and for the 2 water parks (Blizzard Beach and Typhoon Lagoon).
Access to both the Downtown Disney area shops and restaurants is free, but if you'd like to visit special venues such as DisneyQuest or Cirque du Soleil you will need a ticket for those events. Likewise, with Disney's Boardwalk access to shops and restaurant areas are free while there are some clubs that may require an admission fee.
There are many options with Disney park tickets, but an important item to note is that the price to add additional days is usually pretty reasonable. For example, currently a 3 day adult base ticket is priced at $219 (plus tax, etc), while a 7 day ticket is priced at $234 (plus tax, etc). Basically, the price per day reduces the more days you add.
Water parks do require a different ticket unless you add the "Water Parks, Fun and More" option to the tickets mentioned above. If you plan on only 1 visit to a water park or DisneyQuest, then buying a single day admission to those areas is the more economical route. If, however, you may visit both water parks or decide to visit DisneyQuest, Disney's Wide World of Sports or play golf, then the "Water Park, Fun and More" add-on is usually the route to go.
To compare ticket prices and options, you can visit the Disney Tickets page found
here.
Have a great trip!