Santa Margherita Ligure is a good starting point for a beautiful daytrip around the Portofino peninsula. My recommendation is: take an early train to Santa Margherita Ligure, hike or take the boat following the Santa Margherita Ligure-Portofino-San Fruttuoso-Camogli circle, then take the train back to your accommodation from Camogli. The peninsula is full of wonderful hiking trails but unless you are a very fit and experienced hiker, it's better to hike one or two sections, and for the rest of the trip, take the boat.
When you get off the train at Santa Margherita Ligure railway station, watch carefully your belongings. I am not sure what the current situation is but for years there was a group (including women and kids) focusing on tourists, causing some chaos as people were trying to get on and off trains with their luggage and stealing their valuables. So be careful there. Once you walk out of the station, go down the stairway, this will take you to the beach and the center of town.
Santa Margherita Ligure is an old fashioned riviera resort with elegant (and mostly expensive) hotels, restaurants and shops. There are many beaches in and around town, mainly private beaches, but you will also find free public ones. Make sure you have your swimming suit with you. In the center, you can enjoy walking around narrow streets between beautifully restored houses, then head down to the harbour which is full of yachts and fishing boats. In the park by the water, you will see two interesting monuments: the Statue of Cristoforo Colombo, who was born in the area, and the Statue of Vittorio Emanuele II, the first king of united Italy.
The hills around the bay are full of amazing villas, hiding in nice Italian gardens. (I wonder how many million euros they cost… not that I am planning to buy.) There is also a villa and garden that you can visit, the 17th-century Villa Durazzo and its public gardens. It's nice to sit around here for a while and enjoy the magnificent panorama of the coast.
Once you have looked around Santa Margherita, take the boat to Portofino, or you can also take the bus. If you choose to hike this section, you have two choices. You can follow the trail over the hills or just walk on the pavement along the sea which is the easier version. I love both. By the sea, you will see more breathtaking villas and there are many small beaches with crystal clear water so you can combine walking and swimming. If you have snorkeling equipment, take it with you, as there are lot of fish in this protected marine area.