Think beyond the hull, too. For me, one of the greatest things about sailing is that no matter how skilled you become there is always
something more to learn. If you think you've reached the end of what you can learn, you aren't trying hard enough.
In preparation for our Next Boat we obtained our amateur radio licenses. We're now studying for our General Class license and plan to get the Extra Class. My 18 YO niece is also studying for her license as a way of being involved and (from my perspective) becoming a crew member with additional skills for when we bring the boat across. We've been studying along with another SN friend and it is a blast sharing experiences and helping each other.
As I was once told (and is posted in SN somewhere):
Seriously, there is very little you need to learn to get a ham license you shouldn't know anyway to be a self-sufficient cruiser.
For me, the Extra Class license is seriously pushing my mental envelop as math never came easily to me. I'm hoping that what I lack will be made up in the discipline I now have as an adult that I didn't have as a 16 YO. Plus, now I can clearly see a use for it.
And for where you are now in your boating life, knowing the basics of VHF radio and how to properly use it should be on your list anyway.
I'm also taking weather seminars and classes by professionals. Again, something new to learn beyond what is taught in a basic boating class or on The Weather Channel that can be applied not only to sailing itself, but to the radio and just everyday life. I keep a well-marked copy of Reeds Maritime Meteorology
nearby as the same person above told me "Don't just read it, learn it."
Another SailNetter recently attended a 3-day course on dealing with medical emergencies at sea. Something similar might also be on my list for the future.
In the meantime, we're sailing This Boat, having fun applying what we learn and meeting new sailors along the way.