in SJO your options to see things are , Gold Museum, National Museum, Jade Museum, National Theater (and if it sounds interesting go to Mercado Central and have lunch there)
this time of the year, as its our summer here, you might get decent views from Irazu and Poas volcano (Poas is on the way to fortuna so you can do that one then) , from Irazu you probably can see the smoke coming out of its 'brother' Turrialba volcano , who is active right now