Tower of London

From the St James Park west bound district line will take you to Tower Hill Tube station.
You need to walk out of the tub station and walk to the Tower of London Museum.
Admissions to Tower of London for an adult cost 16.50 GBP, however with London two for one voucher you can get 50% discount. The prerequisite for getting two for one deal is that you need to have rail tickets or London Tube day pass. At the ticket window, you need to fill in the voucher and they will give you two tickets at rate of one. One can provide the hotels address where you are staying in as contact address in UK.

There are guided tours available at the entrance of the Tower of London Museum and these are included in the entrance charge.
The Tower of London museum hosts the Queens Crown with Indian Diamond Kohinoor studded in it. It also has some other crowns and jewels on display.
You can take your picture with the Bobby standing in front of the Museums gates.