I'm travelling to Barcelona in May, what should I see and what should I avoid?

Hi :D I'm going on holiday soon to Barcelona to celebrate the quarter of a century that I have graced this planet with my presence.

I've been there twice but only for day trips. I want to know:

- What should I visit/see?
- Where is cheap to eat/drink?
- Where should I avoid?

I already know to avoid Las Ramblas.

Thank you loaf you