There are many starting from Indian Museum, Victoria memorial, howrah bridge, birla planetarium, Dakshineshwar temple, Belur math, eco park, nicco park(its a theme park, so u may also omit this), Princep ghat, jorasanko thakurbari, vivenkananda house, college street, coffee house and the list goes on... so make sure u come to Kolkata with at least 3-4 days in your hands. happy travelling ☺