Top Trails runs Corcovado day hikes and overnights out of Drake Bay with local ICT-certified guides and deliberately small groups. Corcovado requires a certified guide — that is park rule, not an upsell — and the difference a good one makes is the difference between walking through forest and being shown a tapir.
The Sirena station trips are the serious ones: boat down the coast, a full day on the trails, and the best chance in Costa Rica of seeing all four monkey species and a peccary herd.