On our last cruise I had forgotten about signing up for mixology in advance. By the time I remembered all the slots were full. We had no trouble booking onboard after we had eaten lunch (maybe around 1pm). The class we wound up in wasn't even at the times listed online, so I guess they offer more specifically for people to book once on the ship. Our favorite mixology class, however, was over a Thanksgiving cruise 3 years ago. I found an online group for people going on my cruise, and someone in the group had arranged a private class for people in the group. While everyone was still technically strangers, it was fun to kind of "meet" them beforehand and share the class together. Maybe you can look into various social media websites and see if there is a group for your cruise that's planning something similar.