You can also divide and conquer - assuming more than one adult of course. One adult go get the gathering tickets and one go to the appointed location and get the mixology tickets as long as you've agreed on which one(s) you want to do and are flexible with times.
Spa treatments *can* be signed up for on line, though I have never known there to be a problem getting them onboard beyond the "it looks like we'll fill up soon, so you NEED to make your appointment NOW!!!" pressure you get when you ask about them in person. (In my experience, it IS a tactic because I called later that same day to enquire about one and was again told "Yes, we have plenty available. But...they are filling quickly." So, yeah... I didn't book. Waited to get home to the guy I know has a clue what he's doing for my massage.)
Now, if you want the length of cruise Rainforest Pass (much better than the "treatments" IMHO) those do sell out, so I would get that bought quickly.