Hi!
Some top tips from me...
Oyster Card
Have her get an Oyster Card. It's a travel card that you can top up, or load with a weekly pass. You can buy it at any Underground station.
You can use it on trains, trams, the subway (underground) and buses across London.
On 'Pay As You Go' Ypu top up the card with money and you pay less than paying for each fare individually, it will stop charging you if you reach a maximum limit each day (woo hoo, free travel!).
Depending o where she is staying and how much she will travel, it could be worth her getting a week or month travel card. You select if for the zones you will be travelling in and can travel as much as you like within those zones for no extra charge.
Do the Red Bus Tour
There are a few different ones, but all are much of a muchness.
Big Bus Tour
Hop On Hop Off
It is an excellent way to get your bearings in the city, and she can hop on and hop off at various tourist destinations. I do one of these whenever I travel somewhere as it's good to know what is where, and to get around the tourist sights.
Travel on the river
On a nice day she could get a boat down the Thames from Westminster Pier. From there she can wander round Greenwich (where some of Pirates of the Caribbean: On Stranger Tides was filmed), visit the observatory, see the Meridian Line and maybe have a picnic in the park where the Olympic equestrian events took place in 2012.
Check out the museums
The Science Museum, Tate Galleries (Modern and Britain), British Museum, National Portrait Gallery and more are all free.
There's a full list here -
TimeOut Free Museums
Explore
If she wants to see more of the UK, the trains are safe and pretty good. Book in advance using The
Trainline website to get cheaper fares than booking on the day. Oxford and Brighton are all a short journey from London and great for wandering around. If she's feeling particularly adventurous then you can even do a day trip or overnight trip to Paris on the Eurostar from Kings Cross.
Harry Potter
If she likes Harry Potter then a short train ride (or organised coach trip) will get you to Watford, where the Warner Bros Studios are and the Harry Potter movies were all filmed. The studio tour is self guided (get the audio tour, it is really worth it), and you can see all the props and see the special effects from the films.
You're a wizard
Groupon
We have Groupon here! It has really good offers for dining, activities and events. Sign up for the London version. I have done Zoo Lates (adults only late nights at the zoo), the Harry Potter tour mentioned above and West End shows (like Broadway) all for great prices via Groupon.
London Pass
I've never used this as i live here, but it could be an opton for her if she will make enough use of it
London Pass Website
I hope she has a fantastic time.