Getting a walk in Las Ramblas is something really nice, but be careful with your wallet, it is full of thieves. It is not dangerous because this kind of thieves are not violent, they are artists of getting the wallet while you are just walking and without you notice it.
Las Ramblas is full of restaurants and tapas bars. 100% for tourists, expensive and really bad quality. You can take a rest in a terrace and have a drink, but I do not recommend eat there or order a sangria there.
The Gothic area and the Cathedral is fantastic. You can make a walk there and loose your self in all this small and old streets full of life. You can also find there a lot of small cafes where to sit and just let the time pass.