If you have 6 hours or half a day to spend in Southampton, here are some suggestions to consider:
The SeaCity Museum is a great place to learn about the Titanic and the city's maritime history. The visit should take around 2 hours.
The walls around the city have been standing since medieval times and have varying levels of preservation. The walk takes around 1 hour.
The city's Old Town has a variety of interesting buildings, some of which date back to medieval times. Exploring this area can take around 2 hours.
The Solent Sky Museum has a fascinating collection of vintage aircraft. A visit should take around 1-2 hours.
These are just a few ideas to get you started. There are plenty more things to see and do in Southampton!