If you don't feel jet lag, you can visit Placa Catalunya on that day. Then next morning, either get a private tuktuk tour or a guided city tour from https://gowithguide.com/spain/barcelona to get acquainted with the city properly. The next day for Sagrada Familia, you should buy tickets in advance from https://sagradafamilia.org/en/tickets, or else you won't be able to get in. After Sagrada Familia, head to the Gothic Quarter. It’s an easy walk from there.