If you have 24 hours to spend in Londonderry, there are several things you can see and do:
1. Walk along the historic city walls - This will take around 1-2 hours depending on how long you spend admiring the views and taking pictures.
2. Visit the Guildhall - This historic building is home to the city council and has beautiful stained glass windows and an impressive organ. It will take around 30-45 minutes to visit.
3. Explore the Peace Bridge - This pedestrian bridge is a modern addition to the city and offers great views of the River Foyle. Walking across it will take around 20 minutes, but you can also take your time and enjoy the atmosphere.
4. Check out the Tower Museum - This museum has exhibits on the history of the city, including the Siege of Londonderry. It will take around 1-2 hours to explore.
5. Visit St. Columb's Cathedral - This historic cathedral dates back to the 17th century and is one of the city's most notable landmarks. It will take around 30-45 minutes to visit.
6. Enjoy a meal in one of the city's many restaurants - There are plenty of options for food in Londonderry, from traditional pub grub to international cuisine. Depending on where you go, you can expect to spend 1-2 hours.
Overall, you can expect to spend around 6-8 hours visiting these sights, leaving plenty of time to explore the city at your leisure.