Since you're in contact with some of your fellow travelers why not reach out to them about what/how much to bring? You're going to have a guide so this seems like a very well planned excursion so the whole aspect of determing what to do and when is a non issue. Regarding your data plan, contact your carrier and see what modifications to your plan can be made. I don't have a smart phone, but I know that my wife has been able to temporarily alter our normal plan when we're abroad so that she has internet access.
And don't worry about looking like a tourist. You most likely will, especially if you're walking around with a camera and in a group. Who cares? Your group won't and any resident who sees you is someone you'll never have contact with. IOW, enjoy!
You mentioned Barcelona and coincidentally my son is going to be spending his spring semester in the city. We're going to use that as an excuse to fly there in April.