Benin City is inland, but the Nigerian coastline is reachable for a day out. The beaches south of Lagos — a few hours' drive — are the closest options. They are calm, warm, and largely undeveloped.
Akodo Beach (also called Eko Tourist Beach) is the most developed option, with an amusement park, hotel facilities, and a swimming beach. It is geared toward tourists and priced accordingly.
Eleko Beach, near the Lekki Peninsula, is quieter and more relaxed. A small entry fee gets you onto a peaceful stretch of sand with no hawkers or distractions. There are no facilities, so bring your own food and water.
Coconut Beach, near the border town of Badagry about 20 miles from the Republic of Benin, is surrounded by coconut palms and has a somber history as a former slave trade site. The nearby town of Badagry has food and supplies. Bring sun protection — there is little shade away from the tree line.