Sivasagar, better known as Sibsagar, is a district in Assam. Known for its monuments representing the Ahom kingdom, Sibsagar is a must visit if you are visiting the state.
A significant sector for tea trading, Sibsagar is home to numerous tourist spots including Gaurisagar tank which is an artificial tank. There are temples dedicated to Lord Shiva and Lord Vishnu surrounding the tank that are a must visit. Rang Ghar, Shivadol, Ahom Museum and Kaziranga National Park, which is a little further away, should definitely be part of your itinerary.
Another interesting spot is Kareng Ghar, also known as Garhgaon. A local tale surrounding Kareng Ghar states that a secret tunnel here leads you all the way to China! Well, whether or not it does, it's a great spot to visit if you are fond of history, architecture and art!
Sibsagar is perfect for a day trip since you'll be able to pack in quite a number of places. Do keep the National Park for another day since it easily takes up the whole day!