You won't need a guide to hike from the coast road up the Opunohu Valley to Belvédère Lookout. Up and down will take most of a day. It's a level but hot walk along the valley floor and gets steep approaching the lookout. Bring lots of water.
Several unmarked hiking trails lead into the mountains, including one beginning in Cook's Bay and ending on the east coast near the Vaiare ferry dock, another from the southwest coast near Haapiti village across Three Coconuts Pass and into Opunohu Valley. Go with a guide on longer hikes up in the mountains. Moorea-based Tahiti Evasion (tel. 56.48.77; www.tahitievasion.com) has half-day treks to the archeological sites in the Opunohu Valley, across Three Coconuts Pass between the Belvédère and the south coast, and to the Afareaitu waterfall and the pierced Mount Tohiea. Prices range from 4,000CFP to 6,000CFP (US$40-US$60) per person.