Henry- if you're only taking 35mm, the sky's the limit as to what and where you can shoot. Barcelona is a photographer's delight, just be aware that in the Barri Gotic (the area around the old Cathedral) the streets are extremely narrow and the contrast will be very high, as the shadows cast by the buildings can be pretty deep. Barcelona is very safe in general, even Las Ramblas (just be reasonably cautious and you'll be fine).