Nainital is a much bigger, crowded and noisier version of Naukuchiatal and can be reached within couple hours. I made a day trip and had the following activities planned:-
Stroll by Naini lake - The main road is situated along the length of the lake which is ideal for a lovely long stroll by the lake.
Drive upto the topmost view point - See the entire hill station of Nainital as well as the far off Himalayan ranges on a clear sunny day.
Get a little adventurous at the Eco Cave Park - get down and dirty and walk (even crawl) through 6 different types of caves! Some caves are narrow and difficult to walk through and one may need to literally crawl to get through. The air trapped inside the caves make them exceptionally cool and a lot of fun to explore.