What hotels does Costco have available? I am pretty familiar with the area for off site hotels. The area is not nearly as big as Disneyworld so there are plenty of options within a 15 minute walking distance of the park.
Here is a link to Bubbaray's 2011 trip report and she reports that the Best Western Anaheim Inn on Harbor Blvd. is a great place to stay. It is literally across the street from the Harbor Blvd. main gate and it is a nice walking distance to the Anaheim Garden Walk area where there are some nice restaurants, plus there is a Mimi's Cafe up the street, and a Denny's right next door IIRC. It would also be a do able walk from the Disney property hotels, and Downtown Disney if you decide to hang out there.
Other nicer hotels in the general across the streat location are the Marriott, and the Sheraton at the Convention Center, but they cater to business people so not as many or as close breakfast options.
If I was still hell bent on staying on property I would probably check out The Disneyland Hotel as well. It isn't that far of a walk from the park or you can take the Monorail and I believe you can often get better deals there than you can at the Grand Californian from what I understand.