Hawaii Sunday
Stars! You have to go to the observatory visitor center and the observatories. You just have to. If you think the stars are great viewing in your country side/mountain/beach, you haven't been on Mauna Kea. That's why the observatories are here. WOW. Not only are the stars about an inch away (or so they seem), they are nearly at your feet. You don't have to look up to see them, you just have to look... they are all around, even eye level.
Hints: make sure you plan your trip on a new moon or after moonset. Full moons here are gorgeous, but the wash out the stars.
Children under 16 can go to the visitor's center (at 12k), but not the observatory tours (14+k feet).
DRESS WARMLY! It's cold up there.
Volcano! Go to the national park, it's a great and strange landscape. Also, there are some current lava flows. Sometimes you can see them by hiking to them, other times you need to take a helicopter tour (around 200 a person... budget breaker for some).
Richardson's beach. Free :). It's a small (I mean it) black sand beach, but great for kids. Waves are small and you can go snorkeling and see some great fish.
Hint: Go at 4pm. That's when most people are starting to leave AND it's when the sea turtles start to swim up to shore to feed (easy to spot, they swim right next to you!.. just don't touch and disturb them).