First time solo-ish traveling! Looking for Barcelona and Nice recs

Hi there! Me and my sister just booked a week in Barcelona (with a stop to Nice) and I am so excited, but also kind of nervous since it's my first time traveling outside of Canada without a parental figure lol. And since I am older, I am kind of taking the lead on planning. We are both early 20s, have never done hostels before, and feeling kind of naive looking into all of this!

Just looking for any general advice or specific suggestions on places to stay, things to do, etc, for two young women in Barcelona and Nice :)

We'll be there from the end of April to the beginning of May