I was born and raised on Maui, and as we always say, Maui No Ka Oi! so my vote would be for Maui.
Maui has amazing hiking oportunities, with the many trails of the Na Ala Hele trail system (hawaiitrails.org). The website runs really poorly for me, but the Hawaii Department of Land and Natural Resources has a great, if limited site for state sponsored hiking trails. These websites list trails for all the other islands, too. The Haleakala National Park manages visitor centers at the Summit of Haleakala and in Kipahulu. Maybe by the time you get here, the state and county parks in Iao Valley will be back open.
Also, we have town parties around Maui on the 1st, 2nd, 3rd, and 4th Fridays of every month. They're even trying to make 5th Friday happen on Lanai. I find these great fun, with lots of local food and entertainment.
Check out MauiNOW.com for local news and weather reports, but also for info on local social and sporting events.
And never underestimate the fun of the touristy activities. Go to the beach. Snorkel from the beach or on a tour. Drive to Hana (and stop plenty). Attend a Luau (and definitely join in when they ask for audience participation). Ride a helicopter around the Island. Watch the sunset from the beach, or from your balcony, or from the road, or from the top of Haleakala.
Most importantly, remember that what makes Hawaii special is its beauty and culture. So take advantage of it, but always be respectful of the people and the Aina. Leave any place you visit better than you found it.