I don't know if Costco has other ticket packages for DL (I think they do sell the City Pass thing?), but we got the Play 4 days tickets this summer. We've been to DL 3 times (once June, once July, once August) and plan on going in October.
I think it's a pretty good deal, considering what the price of a one-day ticket alone is. After you purchase the ticket at Costco (well, technically, you take the plastic package thing to the cash register, and then after you pay), you go to the merchandise pickup to get the actual tickets. Then when you go to DL the first time, you stop by the ticket booths and give the CM the ticket. He/She will ask for a driver's license or ID and then type your name into the computer. Then he/she will give you a new ticket with your name printed on it and void the one that you gave him.
Then you take your new ticket to the entrance and that is the ticket that you use to enter into DL or DCA. Sometimes they check IDs, sometimes they don't. The first time we used the tickets, they didn't. Second time - my sis and I went through fine, my parents had to get their IDs. Third time - my sis (who was in front of me in line) went through without showing her ID, I had to show mine. So it's kind of random.
I like the Play 4 deal because there isn't a certain # of days you have to use the tickets within. I think for park hoppers, it's within 13 days? For Play 4 days, you just have to use before November 17th (I think that's the date).