Throughout August you will be able to travel throughout the South West Trains network for just £10 return for adults, and £5 for children. Part of their Midsummer Madness promotion.
Available on the majority of Saturdays and Sundays throughout August, passengers will be able to call into their local South West Trains station and pick up a day return ticket between any two destinations on the South West Trains network.
I saw the offer in the local train station yesterday on the Portsmouth to Waterloo service. So that would be excellent value.