A favorite summer activity on the west coast: tide pooling during minus tides. This is when the water scoots back to reveal areas that are undersea all but a few hours a year. I love poking around to get a glimpse of the crazy world under there. No wetsuit or breathing apparatus. No sharks. Thousands of tiny fascinating creatures.
Being a nerdy nature type, I mark the minus tides on my calendar at the beginning of each summer. There are only a few during the day and on weekends, so without planning it's unlikely to see one. I hadn't done it yet this year but happened to check yesterday morning and got to catch one in the afternoon.
Highlight: found 2 opalescent nudibranches. These are extremely cool tiny sea slugs with glowing colors and bizarre shapes. Tough to find. I just stared into various tide pools watching for movement and lucked out this time. Here's a pic, wish I'd taken a video as this one was very active doing acrobatics with the seaweed.
For much better photography and variety, this National Geographic slideshow on nudibranches is gorgeous.