For anyone planning a visit to Spain, I can't recommend Madrid highly enough. I wasn't sure what to expect from the city, but it might be my favorite I've ever been to. Madrid is beautiful, clean, walkable, and very welcoming. The food was all amazing and it seemed like we never had to wait for a table. I had a great time in Barcelona, but I'd recommend Madrid over Barcelona in a heartbeat.
I'd recommend Valencia in addition to Madrid. I can't say I enjoyed Barcelona that much, it felt too touristy. Valencia had a wonderful balance of tourism, low prices, food, and waterfront.
I had almost no idea about Spain before I moved to Europe and soon realised that it is a friendly place with amazing reasonably priced food and some of the most amazing artwork. Every new city has these qualities and adds its own stories and beautiful architecture.
Also Spain is to gin and tonic what New York is to pizza and Australia/New Zealand are to coffee: they did not invent it, but they certainly perfected it. a giant fish bowl of gin mare and good tonic water filled with ice and maybe some rosemary for scent.