#1 Dive, Dive Dive.
Check out some great shore dive sites at :
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands
My favorite sites are all by Wailea (5 caves / 5 graves, Makena landing, White Rock Beach, Ulua Beach etc.). I'd highly suggest a local DM. My fav by far for newbies and experienced guys is Shaka Doug Corbin. I've heard great things about KrisB, Ed Robinson and B&B but no personal experience.
I've done the boat dives and I get better value for my $$ by engaging a local DM to show me around and doing shore dives. Other than a Whale Shark or a giant Manta that you may occasionally see on the backside of Molokini, you can see pretty much everything else from a shore dive off Maui that you will see on the boat dives.
#2 Do the road to Hana (most of a day trip). especially if you are going to have an overcast day.
The road to Hana is a neat experience but make sure you go buy a CD and listen to it or you will miss so much. You can pick them up at just about every souvenir shop. Lots of 1 way bridges, breathtaking scenery, great eucalyptus trees, etc. etc. Very windy road so don't let the kids play on the DS on the way down and pack some gravol for people in the back
#3 Top of Mountain Haleakela
The trip to the top of Haleakela Mountain is pretty cool and if it is clear and you are there at sunrise, its supposed to be spectacular. Bring a heavy jacket as its cold up there!
#4 Hit the lava flows on the way to Black Rock Beach (south east side of island past Wailea).
#5 Take a surf lesson near Kehei
#6 Head to the North Shore (not far from the airport and you will pass it on your way to Hana). Stop here and watch the local surfers, wind surfers, kite surfers etc. They are amazing and the waves will be big on this side of the island in January.
#7 Go to a different beach each day. We still do this every time in Maui and its really what makes Maui special to me. Different reefs, different beaches.
#8 Watch the whales. From every beach, from your balcony.
#9 Book your return trip for next year...