National museum, Victoria memorial, birla planetarium, fort Williams, science city, Alipur zoo, botanical gardens, Belur math, St Paul's cathedral, kalighat temple, dakshineswar temple, national library, marble palace, jorasanko thakur bari, Vivekananda birth place and museum, ganga view organized by the government.
First tell me, what interests you more, because Kolkata can surprise you differently...
It has British architecture, religious places, natural resources, mordern amenities, heritage buildings, years old market, street shopping....and my list will continue. And the time when you are coming, you can even go for various fairs.
In Kolkata, you should visit the Victoria Memorial and Indian Museum. That apart going to Eco Park in Rajar hat would be a good place. Go to Park street for food, like Peter Cat, Olypub, or for the famous biryani i would suggest going to Aminia, or Royal Indian Hotel for the Alu-less variety of it.
3-4 days will be enough. There are a plenty of must visit places in Kolkata. Victoria memorial, Howrah bridge, Belur math, Indian Museum, Birla planetarium, Dakshineshwar temple, college street, Vivenkananda house, Jorasanko thakurbari etc. are few of them. If you have time browse the Park street, Gariahat market. You can go to Bhohohori manna if you want to have a flavor of authentic Bengali dishes. Though it's a bit expensive. happy travelling. ☺
Welcome to Kolkata. Places to visit-
Victoria Memorial
Prinsep Ghat
Maidan Dakhineshwar Temple Kalighat
Eco Park Esplanade Deckars Lane( For best street foods)
Kumortoli
Mullick Bazar( Biggest flower market)
Coffee House
Etc.