AFAIK, you can still buy a drink package later in the cruise if you decide to. My next cruise is only a 4-nighter, so I won't be able to check for sure, but I expect it's still true.

The dining packages usually apply to all of the upcharge speciality restaurants. You can go on whatever night you wish, just make reservations. I did not buy the dining package on my last Allure cruise, but our friends did. I don't remember any tricky restrictions. If you're only going to one specialty restaurant, then the dining packages aren't worth it. They will save you a couple of bucks if you plan to visit more. If you want to get a feel for Wonderland, check out my Quantum of the Seas review and scroll through to the dining section.

Stateroom configuration and furnishings will also have more in common with Quantum than Harmony's sisters Allure and Oasis, as I understand it. (Please note that FT rules require that I advise you that I am a contributor to the blog linked to here)