One of the 10 best dive spots in the US is, surprisingly, right here in li'l ol' Santa Rosa. The hourglass-shaped, 70ft-deep Blue Hole is 138ft in diameter at the surface and has a dive shop (open Saturday and Sunday; rentals only) alongside. Fed by a natural spring flowing at 3000 gallons a minute, the water is both very clear and pretty cool (at around 62°F/17°C). Divers should apply for a 10-day certificate ($20).
Kids can check out the neighboring Wibit Water Park ($10), a good spot to cool off with slides and an inflatable obstacle course.