Normally I'd not suggest lining the pockets of TFL more than you already do, but in this case, I think it's perfectly acceptable to give them £20 in exchange for access to one of London's most famous closed-down tube stations.
Two weekends' worth of tickets have just gone on sale (2,520 in total) across 25th - 27th November and 2nd - 4th December, with each 40-minute tour also receiving a one-day pass to the London Transport Museum, too.
While the tickets are pretty pricey at £20, it's a great look at something only the sewer rats have had the luxury of using since it closed in 1994. You can lose hours reading up on its history at Wikipedia, or checking out SilentUK and their daring escapades sneaking into the city's disused tunnels and stations. [London Transport Museum]
Image Credit: SilentUK