Nainital is the lake district of Uttarakhand. It is famous for a few things:

Surrounded by hills and covering a natural lake.

All season amazing weather. Cool in summer and chilled snow in winter.

Boating is common tourist interest.

China peak/naina peak trek is most popular.

Nearby visiting place like kunjakharak, kilbury, bhimtal.

Fashion and sports clubs.

And there are many more attraction in Nainital.