Molokai was an ancient religious center, of which Ili'ili'opae Heiau serves as a stirring reminder. Kahuna (priests) ritually sacrificed humans at the altar (making this a luakini heiau). On the day I went, I could not hike through the brush to view the site from the top but nevertheless felt its power. It may not be the most action-filled excursion, but if you are interested in the history of the islands, then the 10 minute hike is worth it.
Off Kamehameha V Highway (450), around mile marker 15 by Mapulehu Bridge, 1 mile west of Mana'e Goods & Grindz, you will see on the mauka (mountain) side, a gate with a sign saying "Heiau." The shopowner at Mana'e can help direct you if lost.
Park in any nearby turnout, walk to the gate and pass through or around. About 5 minutes in, you will see one sign at your left for "Ili'ili'opae Heiau Sacred Hawaiian Temple", then farther up, an identical sign just before the clearing by the house. This sign will direct you left into a (dry) riverbed. From there, you will see three walls forming the base of the heiau. You can hike up and around to view the entire rectangular platform of stones, which measures around 320' x 120', but take care not to step on or disturb the stones.
Bring mosquito repellent (red ants and insects abound, partly due to the mango trees lining the path) and wear sturdy shoes. Various websites recommend you contact the landowners, but visitors are welcome so long as they are respectful.