I’ve come across a special promotion on the National Rail Enquiries website where adult Oystercard holders (and Freedom Pass holders) can escape from London for very low fares. £5 will get you a day return to many places just outside London while £20 will get you a saver return to places like Cardiff, Liverpool and Great Yarmouth. First Class fares are also available.
Most train companies operating out of London seem to be involved apart from Airport links, East Coast, East Midland Trains, Southeastern and Virgin*. The promotion runs from June 26th to July 25th. You need to print off a voucher and take it and the Oystercard to a booking office to get the discounted fare. Full details can be found here on the Days Out Guide website while the voucher is available here on the NRES website.
* Note that London Midland are taking part so you can use their trains to get as far north as Liverpool!