Do take time before you travel to research and list the things you really must see - there is so much in Barcelona that you can literally be there for a month and still have things on your list. Don't go for 2 nights unless you are just there for the nightlife!
Breakfast in the Barcelona hotels is available quite late into the morning. Generally breakfast is served somewhere between the hours of 07:00 and 11:00, but some offer breakfast up till 11:30 and later at weekends or on Bank Holidays! If you're an early riser and your hotel is central, why not take an hour or two to wander round and get a feel for the city and then, having worked up an appetite, come back to the hotel for a late breakfast. You probably won't need lunch because in the 3 star and above categories you will get a full buffet breakfast. If you are a late riser (and Barcelona stays alive well into the early hours) you can enjoy a lie-in yet still make the most of your hotel's breakfast.
Take the tourist buses that tour the city - they can be boarded anywhere on the 2 routes (blue and red) and with a day ticket you can get on and off at as many sites as you wish (the buses run every 20 minutes). The routes offer great flexibility and whilst on the bus you will be treated to a commentary by a guide.
There are hundreds of small, unique and very interesting shops in the many alleys of the Gothic Quarter. If you see something you like but don't want to buy it just then, make absolutely sure you know the address of the shop and how to get there, because trying to find it again two days later can be very frustrating. If you're rushing around to get gifts on your last day, you probably won't find the shop again.
Whether you are there in winter or summer it is always worth a walk along the beachfront. This is generally where you'll find the finest fish restaurants (although they are expensive). The beach area provides an interesting contrast to the built up and very busy city centre and yet it is only about 25 minutes' walk or a short underground journey from the Ramblas and Gothic Quarter.
Eating out in Barcelona can be expensive, especially in the evenings, so take advantage of the good lunchtime deals that many restaurants offer. Many have 2 or 3 course meals at a fraction of the price you would pay in the evening. In particular, try the soups: they are very filling and hearty but just right for a "lunch on the go."
The Ramblas is a very busy and lively street so if you ask for a Ramblas view room and are allocated one expect a bit of noise at all hours. Indeed if you are looking for a quiet hotel location you are best not to be on the Ramblas itself but in the Gothic Quarter. Expect generally small hotel rooms here due to the age of the buildings.