Longtail boat day trips to Long Beach (with snorkelling, fishing and swimming thrown in) are the main Koh Rong activity. Pretty much every place in Koh Tuch village organises them for about US$10 per person. For US$20 you can stay out longer and add an evening excursion to see bioluminescence.
It's also viable to walk one to two hours from Koh Tuch to Long Beach via a rigorous jungle track, but be aware that it involves scrambling and is definitely not flip-flops territory. Don't do this at night or alone.
For an adventurous day out packed full of scenery, it's possible to organise a boat to Sangker village on the island's east coast (US$30), then hire a moto (motorcycle taxi) driver to whiz you around the island. Adventure Adam is a popular tour guide.
There’s good snorkelling around the island and resorts rent out gear for about US$5 per day. The best snorkelling around Koh Tuch is in the cove in front of Treehouse Bungalows.
Several places hire sea kayaks for about US$5 per hour for a single kayak (US$8 for a tandem). From Koh Tuch Beach, it’s a 30-minute paddle out to Pagoda Island, an idyllic islet topped by a wat, just offshore.