I started to write a long look at reserving activities, then I realized it was just a bunch of paragraphs that really didn't say much...
...so here it is in a nutshell. There really is no hard and fast rule for reserving activities when traveling, but if it were me, I'd pick the thing or two I really want to do, and reserve it in advance so I know it's locked in, leave some open time in case I get there and discover something interesting I want to do, then go from there.
What brought this on is one little thing... Christmas is coming. Quite often I get lots of calls around 8pm looking for a dive tomorrow morning, it seems to happen almost every day during Christmas week - Sometimes I can make it happen, oftentimes I can't, it's much more difficult at that time of year.... I suspect this is true with most dive operators, especially during the busiest times.
In Kona, our absolute busiest time for dive and other water oriented operators is probably Christmas week. Depending on what day Christmas falls on the calendar, it can go from slow/steady to crazy/busy overnight around the 18th-22nd of December and continue on 'til a few days after the first of the year. My guess is that at least one or more of the most established dive companies will probably have their entire holiday schedule nearly completely set by the 2nd or 3rd week of December. I may not be there yet, being comparatively new to some, but I expect I'll be filling up a good portion of my seats in advance this year.
... So remember, the early bird catches the boat (or something like that). If you are heading to a tourist destinaton this holiday season, it might not hurt to schedule some of your priorities in advance, especially if you have a particular provider in mind.
Above is a picture of an Ornate Wrasse (Halichoeres ornatissimus). In the aquarium trade we'd normally see these show up as a "Christmas" wrasse, but there is another wrasse that occasionally show up when a Christmas wrasse was ordered that Hoover's book has with that name (Get 5 books, you'll probably see 2 names on several fish, scientific names even seem to change every now and then as species classification evolves). Oooohh.... Christmas wrasse, how apropos, considering this is sort of a Christmas topic.