MacKenzie State Park (map), Isaac Hale Beach Park and Pohoiki Black Sand Beach (map) are about a 30 minute drive away to the south.  All three of these places are nice places for a picnic.  The water at these parks is rough and not suitable for swimming. 

Kalapana Black Sand Beach (map) is a bit further to the South.  Park at the same place as the Kalapana Night Market, but then walk toward the ocean rather than toward the market.  The entire area is covered in a lava flow from 1990, so it is a great place to see how the island changes after a flow.  You can clearly see the regrowth of the vegetation and erosion along the coast.  It is a nice place to visit if you go to the early to the Night Market. 

Volcano National Park (map) is also about a 30 mi. away to the South, but because of the Kalapana flow, you have to go North (toward Hilo), then turn back to the South to get there.  This is THE PLACE to go see the volcano up close.  The viewing area overlooks a huge caldera and has lots of great displays regarding the history and research regarding the volcano.  There are also monthly Hula performances at the Park.  This is a National Park, so there is an entrance fee per vehicle.  Check the Park’s website for events and entrance fee information (map) 
 

Punaluu Black Sand Beach (map) is further to the South along the same road as Volcano National Park.  This beach is good for swimming and watching sea turtles.  If you are driving even further South or aroun to Kona, this is a decent place for a restroom break. 
 
Punaluu Bakery (map) is a great place to stop.  The baked goods from this remote bakery are sold all over the Island.  Stop and grab a snack.  
 
South Point (map) is the southernmost point in the United States.  It has tall (~20 ft) looking out over reasonably deep water.  As a result, many people jump off for the thrill.  There is a ladder built into the cliffs to climb back up.  Be cautious though.  There are NO life guards or emergency responders anywhere near here, and the water can get rough. When we go, this is always a stop on our trip to the Green Sand Beach. 
 
Green Sand beach (map)...this is the place.  It is very near South Point, but you have to park and hike in.  The hike is about 3 miles, with no facilities, lots of sand and rocky roads.  But it is well worth it. A swim in the nice refreshing water at the end of that hike is amazing.  The view is like nothing you have seen before.  Cliffs, green sand, red sand, black rocks, and beautiful blue water. Bring sunglasses (to keep the blowing sand out of your eyes) and a good deal of water. 

Carlsmith Beach Park (map) is one of our favorite beaches in the Hilo area.  It is safe for snorkeling as long as the surf is not high.  The water varies in clarity a bit due to fresh water flowing into the area.  The park has lots of shade and grass for relaxing, as well as bath rooms for changing. 
 
Onakahkaha Beach Park (map) is a popular beach park for folks with small kids.  The park again has lots of shade and grass for people on the shore, and a very sheltered swimming area for kids and adults.  
 
Honoli'i Beach Park (map) is great to watch surfers.  The park has a grassy side which is mostly sunny, and another side that more shaded but changes over time.  Sometime this area has black sand and other times it has fist sized or bigger rounded lava rock.  The current can be dangerous, but there is a life guard station. Parking is along the road and can be tight.  Also, be aware that you have to walk down a rather steep set of stairs to get to the park from the road. 
 
Rainbow Falls (map) is a beautiful and conveniently located water fall very near downtown Hilo.  From the parking lot, you can walk to a viewing area overlooking the falls, or up the hill to look down from above the falls.  It is worth the walk up the hill to see the beautiful, huge banyan trees. Many people also swim/wade in the river over the falls, but every year tourists die here because of flash flooding. 
 
 Hilo Downtown (map) has small shops the restaurants and a few museums.  There are also frequent cultural events.  Check here for the calendar of events. 
 
 Panaewa Zoo (map) is a small, intimate zoo.  The animals are well cared for and close to the visitors.  The zoo is free, which is hard to beat! 
 
North of Hilo:  Akaka State Falls Park (map) has a lovely path to walk from the parking area to see the falls.  The falls are quite tall (reportedly over 400 feet) and the path is surrounded by beautiful forest.  
 
 There are two privately owned botanical gardens north of Hilo.  Both charge for admission and neither is cheap, but if you love beautiful trees, plants, and flowers, you need to walk through at least one of these gardens.  Both gardens are quite large.  If you are actually looking at the beautiful scenery, it will take you several hours to see either.  Also, both are an active walk, or even a hike in places, due mostly to the hilly landscape. 
 
o Hawaii Tropical Botanical Garden (map) is closer to Hilo, and in our opinion, is the prettier of the two.  However, it is also smaller.  Another advantage of this garden is its position along a beautiful scenic drive with places to stop and walk or look at the rivers, falls, and bridges along the coastline. The paths in this garden will lead you through ferns, bamboo, flowers, and a fair number of large trees.  The enormous mango tree is particularly impressive. o Bontanical World Adventure (map) is a bit further north and seems to be significantly larger and (the last time we were there) had a less cultivated, more natural, feel about it.  The flora on display here are more spread out but just as impressive in their way.  What stood out here was the colorful Rainbow Eucalyptus trees.  This garden also has a zip line tour, but we have not tried that.