Many things for solo female travelers to do that are safe. I'll give some general suggestions not knowing where you will be staying. These are safe things I like when I have made a solo trip or have a few solo days on the island.
First get a car, many of us use www.discounthawaiicarrental.com/… for our cars. Compact or economy is all you will need.
Morning snorkel sail.
Depending on where you stay I'll suggest beaches, for morning beach time. so get back with us.. Maui has wonderful beaches.
Walk the Kapalua Costal Trail, beautiful scenery. Trail starts at Kapalua Bay.
Iao Valley State Park
Lavender Farm (you can walk the floral gardens, no fee).
Tour the winery.
Drive to the summit of Haleakala during the mid morning, out of this world experience.
Stroll and shop Paia and Makawao
Stroll the boardwalk along the hotels/resorts in Ka'anapali, visiting the whalers Museum and shops in Whalers Village.
Stroll the boardwalk along the resort in Wailea.
Head to the huge Swap Meet in Kahului on Saturday 7am-2.. fun things to buy for your self and friends.
Go to Lahaina (daytime) visit the Banyand Tree, pick up a walking - self tour map at the Old court house of the Historic Lahaina area.
Take a lesson: either surf or Stand Up Paddle , go out with a kyaka group..
Let us know what type of activities you like and we'll have more suggestions.