By Sea - The Molokai Princess makes two round trips between Lahaina, Maui and Kaunakakai, Molokai twice daily except on Sundays when it goes only once. The ferry leaves Molokai for Maui at 5:15am, arrives at 7am and returns to Molokai at 7:15, arriving at 8:45am. In the afternoons it leaves Molokai at 4:00pm, gets into Lahiana at 5:30, leaves Lahaina at 6:00pm and reaches Molokai at 7:45pm.
So...if you come from Maui on the 7:15am boat and return at 4pm you have 7 hours to bike around. If you're hardcore, you can see most of the island. If you're a couch patato you can cruise around town and perhaps go two miles east and have lunch at the Hotel Molokai. If you're somewhere in between, a ride east to the 20 mile mark and back with lunch at Mana'e Goods and Grindz (16 miles east) will ensure a good nights rest.  
The trip coming from Maui is usually smoother as the wind and swells go with the ferry. Heading into the wind is rougher so Dramamine may be your drug of choice. Call (808)662-3355 for reservations or visit http://www.molokaiferry.com/ .

By Air - Call Mokulele Airlines at (866)260-7070 http://www.mokuleleairlines.com/ or Island Air at (800)652-6541 or visit http://www.islandair.com/  to make reservations. Flight times from both O'ahu and Maui are about 25 minutes each way. Flights leave throughout the day but go online early to get better prices.

Paragraph.