When are you cruising as parasailing is seasonal.... Mid-May - Mid-December. The migration of the Humpback Whales through the Hawaiian waters prevents year round operation. Of course if you will be there in Whale Watch season that would be my first choice for an activity!
If you intend on looking for something to do right "in" Lahaina I would suggest doing the Historic Walking Tour of Lahaina! It is interesting and educational.
http://www.hawaiischoolreports.com/lahaina/lhistorictour.htm
What most folks do "in" Lahaina is shop, eat and drink!! Front Street is filled with shops and art galleries and it is very easy to spend the best part of an afternoon just browsing the shops and people watching. Be prepared to be part of the "herd"!!
If you are planning on doing a luau and your schedule will accommodate it, choose the Old Lahaina Luau. It is considered the best in the islands.
For a driving tour, my favourite is to head Upcountry! The slopes of Haleakala offer wonderful views, the terrific little town of Makawao, and the Tedeschi Winery to name only a few highlights! If you don't have time for Upcountry visit Iao Valley.
Enjoy your visit to Maui, my favourite island get away!!!