The largest island in the archipelago and the most visited by those who want to stay overnight rather than just do a day trip. Isla Grande has a few small guesthouses and ecolodges set among the palms and mangroves, and its beaches are quieter than those close to the main tour boat landing points.
Walking trails cut through the interior, and the mangrove areas around the island's edges are good for birdwatching — herons, frigatebirds, and various waders are regularly seen. The snorkelling off the beaches is decent, though the best reef sites require a short boat ride to the clearer waters around the smaller islets.
If you want to experience the Rosario Islands as something more than a day excursion, Isla Grande is the obvious base. Book accommodation in advance during Colombian school holidays and on long weekends.