The Big Island of Hawaii is one of the best places I've traveled for varied, accessible, and fun things to do, not to mention the great food. The Big Island is, well, big, and has everything from arid desert on the Kona side to one of the wettest places on Earth on the Hilo side, ranging from 28" to 128" of rain a year. It's a great place to take kids and for anyone who loves touring and learning. I'm not a water/beach person (surprise!) and am not a huge fan of sit-on-beach tropical vacations, but the Big Island blew my expectations.
Get a car. You can stay on the tourist/retirement Kona side, which is hot and dry, or on the wetter locals' side in Hilo. Familiarize yourself with a map, also below, of where everything is since it can take up to 2 hours to get from one thing to the next. It'll take a little juggling if you're short on time but most is do-able in 3 full days if you plan well. I did AirBNB both times for ~$100/night.
If you couldn't tell, 'Akaka Falls is on the wetter Hilo side. It's a paved 0.5mi round-trip walk to the falls, which is very often socked in with fog and clouds. Bring a rain jacket! It clears up enough to see beyond the railing.
Hawaii Tropical Botanical Gardens
The gardens are also on the Hilo side and can be done the same day as 'Akaka falls. It sits on 40 acres on the shore with more than 2000 species of plants on a 1.5mi paved walk through the forest. These cute things were just a few inches tall! Mosquitoes abound.
Kona Coffee Plantation Tours
There's multiple Kona Coffee plantations that offer tours. All are free and come with free samples. My favorite, and the most in-depth, was the Greenwell Coffee Plantation. All the Kona coffee farms are along a 20mi stretch of land along the Kona coast and must be grown here to be labeled pure Kona coffee, versus a "Kona Blend" or other jargon.
Big Island Bees
10min from Greenwell Coffee is the US's largest organic bee farm. That title is due to being on a contained island--since bees run "free range," most farms on the mainland can't guarantee that bees stay on organic land, and the bees aren't active all year 'round like in Hawaii. While many places get only 50lb of honey per hive per year, especially in cold climates, a hive in Hawaii can yield 200lb/yr.
The Mauna Kea observatory has an awesome stargazing program every night at the visitor's center starting at 6pm. While you can't get to the top of the summit at 14000ft where all the huge observatories are without some serious 4WD, you get a splendid view from the visitor's center at 9000ft. Pretty much every night the fog and cloud cover drops below the 9k mark at or after sunset. We've been to other dark sky places like Big Bend on a new moon, but the clarity from Mauna Kea surpassed even that.
When you drive the southern route from Kona to Volcanoes National Park, you look out over South Point, the southernmost point of the US. You can take a 12mi one-way slow drive on a shared 1.5-lane-wide road down to the point, which in all honesty wasn't worth it, except to say you stepped there. So here you see it.
Manta Ray Night Dive
There's a ton of outfits that do night manta ray snorkel and scuba dives off the Kona coast. We did one through Kona Ocean Adventures, which was a great outfit. Unfortunately, our evening consisted of unusually high 10ft seas and high winds. I didn't get many photos, and the mantas stayed close to the ocean floor instead of being tossed around on the surface with us, so I've a lack of photos. I've got it on good word from a dive instructor that it ranks in the top 10 of 400 different dive locations she's done.
Hawaii Volcanoes National Park
A must-see! I've got a dedicated blog post for Volcanoes NP!
The food blew me away on the Big Island.
Check out my article on Hawaii Volcanoes National Park!