Sindy, your headline confused me... "Northern Spain" is considered to be Basque Country, Cantabria and Asturias, not Catalonia :)) (http://bit.ly/1LeTaQm)
There are so many different things to do/see that it's difficult to reduce to mention a few, especially being a local and not knowing you and your interests -those mentioned above are too general, sorry!-, so it's best if you have a look at these websites and see what catches your eye: www.catalunya.com and www.infocatalonia.eu and for Barcelona especifically: www.barcelonaturisme.com and www.timeout.com/barcelona
Costa Brava has many beautiful picturesque coastal towns, Cadaqués and Tamariu being two of them. This is a video showing different spots: http://infocatalonia.eu/w/AioW1 and this is a slideshow which helps identifying some of them by name: http://infocatalonia.eu/w/ri7ys If anything, I personally would avoid Lloret de Mar and Calella as they're typical destination for rather unruly visitors (kind of spring-break all over!) It's sad because the towns have wonderful spots and locals are very welcoming, but it does not pay. Please note there are two towns sharing the name Calella and they're very different. Calella de Palafrugell, a bit north from Calella, it's a lovely town I visit often.
Since you're renting a car -otherwise you wouldn't be able to move so easily and freely in that area- I suggest you plan a route and visit these two and other towns in Costa Brava. There is a bus network and train of course in the area, but hopping from one town to the next at convenient times for your own schedule can be a hassle.
Since you mention nature and hiking, you might want to have a look at these brochures freely offered by the Catalan Government which aim to present the many things one can do in this little nation while on vacation: http://www.act.cat but first I advice having a look at this map so you can understand where are located the different "vegueries" -sort of administrative divisions of the territory (oddly referred to as 'tourist partners' at the web!): map of Catalonia -click on the white list icon on the top left -underneath the label "Map of Catalonia"- to unfold the names of the different areas.
As per references to La Molina (not Molinos) and Espot, both are tiny towns in the mountains famously known for their ski facilities -not too useful in the middle of summer I'm afraid. Note though they're 100 miles apart! Yet both are located in superb nature areas. If you visit La Molina I advice to detour to the Valley of Núria, which has a sanctuary honouring Our Lady of Núria and Saint Giles. Have a peek here: http://infocatalonia.eu/w/nIprg and for text-based info: http://www.valldenuria.com and also in Wikipedia. And near Espot you should visit the National Park of Aigüestortes i Estany de Sant Maurici (I know, long difficult Catalan name to pronounce, listen here to a short version of the full name). Have a peek: http://infocatalonia.eu/w/3jo1E and visit this website for text-based info: http://parcsnaturals.gencat.cat
Despite I said earlier there are too many things to simplify into a short list, travelling with teenagers I would like to point out Besalú (http://infocatalonia.eu/w/2MR8p) and the Route of the Castles of Sió (http://infocatalonia.eu/w/Rj0A8), the later being quite tourist-free if you catch my drift.
Enjoy!