Done that.
We had an excellent time just mooching around from bar to bar, visiting all the sites / beach / outside eateries / markets.
Basically just wandering around in a “non restrained” fashion in the warm taking it all in, with added drink duties /bars / clubs in the evening of course. Great times with great friends.
Watch out for the hookers at the bottom of La Ramblas after midnight.
Edit: I agree with Mackem – Barca doesn’t contain Europe’s top level nightclubs or anything like that, but of you want a decent bar with shots beer and over the bar snacks thats no problem there’s plenty of those.