You're close enough to Sagrada Familia to walk from where the bus drops you. I'd head there rather than down the Ramblas.
Again, not too far from Placa de Catalunya is probably the best tapas bar Ive ever been to. Cerveceria Catalana on
Carrer de Mallorca in Eixample is a great place to waste an hour or three. Full of locals ( you might have to wait) with stunning good and cava . I've been here a couple of times, last time I actually stayed rather than go to Camp Nou to watch the football.
Otherwise, again in walking distance, is the Picasso museum or just meandering up and down las Ramblas or you might people watch in the W hotel . The port is also very nice .