My favorite 6day/5 night Disneyland vacation itinerary consists of a 3 day park hopper ticket and a day at Universal Studios Hollywood.
There is a shuttle service that will provide round trip transportation from your Disneyland hotel (both on and off property hotels) to Universal Studios for the day. In my experience, Southern California Gray Line provides the best round trip service to Universal Studios. You may purchase their shuttle service when buying your Universal Studios tickets online, or directly from the Southern California Gray Line website.
Here is a list of Anaheim hotels that they serve, so check before you book!