The city has a number of heritage buildings of great value as well as some modern museums and parks, so if you're looking for stuff to do in Vilnius you're sure to find a range of possibilities.
Start your visit to Vilnius in the old town, which is the first of the essential places to visit in Vilnius. It is the largest and best preserved medieval quarter in Eastern Europe and was declared a World Heritage Site by UNESCO in 1994. It is composed of 74 districts, with 70 streets and lanes and a total of 1487 buildings. There is an interesting mix of different styles: Gothic, Renaissance, Baroque, and Neoclassical, so if you're interested in architecture, you'll find plenty of Vilnius attractions here.
Pilies Street is the main artery within the old city and the center of commercial life. In the main town you may follow Gediminas Avenue, which leads to the Piazza Duomo. Other essential things to see in Vilnius include the Vilnius Castle complex, a group of buildings originally meant to guard the city and which are also of cultural and religious significance. Here you'll find interesting attractions in Vilnius like Gediminas Tower, Cathedral Square, the Palace of of the Grand Dukes of Lithuania, and the remains of several medieval castles. The House of the Signatories, where Lithuanian's independence declaration was signed in 1918, is now a historical site and a visit here should be high on your list of what to do in Vilnius.
Cultural Vilnius activities include the Museum of Genocide Victims, National Gallery of Art, the National Museum of Lithuania and the Jonas Mekas Centre for Visual Arts.
Ideas of more stuff to do in Vilnius and a list of Vilnius attractions and Vilnius activities can be found on Minube..