If you have 6 hours or a half day to spend in the town of Greenock, here are some suggestions:
1. Visit the Custom House Museum - this museum showcases the history of the town as a port and has interesting exhibits on shipbuilding and trade. Estimated time: 1-2 hours.
2. Explore the waterfront - take a walk along the Esplanade and enjoy views of the Clyde estuary. There are also several statues and monuments to see, such as the Scott monument. Estimated time: 1-2 hours.
3. Visit the McLean Museum and Art Gallery - this museum has a wide range of exhibits, including art, local history, and natural history. Estimated time: 1-2 hours.
4. Go shopping - there are several shopping areas in the town, such as Oak Mall and The Waterfront Retail Park. Estimated time: 1-2 hours.
Overall, you can choose which attractions you'd like to prioritize based on your interests and available time. Greenock is a small town, so it's easy to get around and see multiple things in a short amount of time.