First stop after getting off the tender is the Old Lahaina Courthouse where you can pick up a free "self-guided walking tour." Lots of places within a short walk to visit. There is a lot of shopping nearby.
Maui Ocean Center Rating:
We took a city bus out to the Maui Ocean Center ($2-$3)- it is a short walk to the bus stop and the bus takes you right into the parking lot. It is a first rate aquarium and a great place to visit. There are nearby restaurants and shopping here as well. Taking the bus out saved a lot of money over what the cruise line wanted.
Right outside the cruise ship we found a tour operator offering a 4 - 5 hour trip to the farmer's market, the Volcano, the orchid farm, the macadamia nut farm, the Liliuokalani gardens, the King Kamehameha statue,and the black sand beach. This was much cheaper (and more stops) than through the cruise ship and they promised to have us back in time. They also gave us flexibility in the amount of time we stayed at each place and actually added the black sand beach to the itinerary when my daughter asked.
Relaxing day at the beach. Crystal clear water. Shallow with no waves you could walk out a quarter of a mile. We had stingrays swimming with us. The only draw back was that it is a coral beach and you MUST have swim shoes or you will tear up your feet.