Without the usual hectic pace of the city, Sunday is actually the most pleasant day to explore the city,. Little to no traffic !! All the sights, museums, restaurants, shopping areas are open on Sundays. The Old Town (Casco Viejo) hosts a artisans/flea market in the main square. It is the perfect day to explore the Old Town and then, late afternoon, weather permitting, stroll down the bayside promenade and Fish Market - grab some ceviche and chill with the locals and families enjoying their free day. Great scene.