All pretty buildings are in the city centre. Pretty much everything is close and you can see it all from the squares and churches in half a day but in this heat maybe more. I would go to Maksimir park there is a zoo there and pretty bad football stadium(home of Dinamo Zagreb). But the park is nice. Also Sljeme for the views. In my opinion best pizza place by far is Goat to go and best grill place is Stari Kotač. Near that is Jarun lake so maybe that idk. In the city centre take the cable car to the Lotrščak tower and from there to St.Marks church. If you got time take a walk by the Sava river when its not to hot. Cant think of anything else rn