An Andaman trip has more moving logistics than most Indian destinations, mainly because a chunk of it runs on inter-island ferries rather than roads, and that's the part worth understanding before you land in Port Blair.
Most itineraries start in Port Blair regardless of where they're headed next, since it's the only airport with regular mainland connections. The Cellular Jail's evening light-and-sound show is worth the first night, and Corbyn's Cove Beach is a reasonable stop if there's a spare afternoon before onward travel.
Havelock Island — officially Swaraj Dweep — is reached only by ferry, a crossing of roughly two to three hours depending on the operator. Government ferries are cheaper but limited in number and sell out early; private operators like Makruzz and Green Ocean run more frequently and are worth booking a few days ahead, especially between December and February when demand peaks.
Radhanagar Beach, Beach No. 7 on Havelock, is the reason most people add Havelock to the itinerary in the first place — fine white sand and a shallow, calm entry into the water that works as well for a family with young kids as for anyone there just to swim. Elephant Beach, a short boat ride from the main jetty, is the better stop for snorkelling given the coral just offshore.
Neil Island — officially Shaheed Dweep — is a smaller, quieter add-on reached by a short ferry hop from Havelock, worth a night for Bharatpur Beach and the natural rock formation at Laxmanpur Beach known locally as the Natural Bridge. Seven nights is a realistic minimum to cover Port Blair, Havelock, and Neil without the ferry schedule dictating the whole trip; we build the ferry bookings into the itinerary upfront, since that's the piece most likely to go wrong if it's left until arrival.