What to do in London with someone whos already been there?

I'm just back from London where I go every year and a thing I had never done is seeing London from a skyscraper. In the City, the Walkie Talkie has a wonderful view all around and it is free! You just have to book. https://skygarden.london/  

Bon voyage with your aunt!

As I'm a North London girl, I would suggest checking out Ally Pally (Alexandra Palace).  They always have special events on http://www.alexandrapalace.com/park/.  Sticking to the 'norf' London theme I would recommend visiting Highgate Village Market  As a kid I spent many hours roaming Highgate Cemetery (Karl Marx is buried there). Kenwood House is right on the border with Hampstead Village and offers wonderful summer concerts on the grounds (you can order traditional hampers to be delivered there from Harrodsand other places).  My home town is Mill Hill, lots of places to walk and see some older buildings http://www.millhill-hs.org.uk/walk.html, and some nice pubs (Adam & Eve and Rising Sun).  Close to Mill Hill (Colindale) is the Royal Air Force Museum London

I've been to London 20+ times and I always find something new to do.  The Sky Garden is an excellent suggestion, you really should book ahead, I think it's roughly three weeks in advance.

Eltham Palace and Gardens is just a short train ride outside the center, it's a beautiful art deco palace.  Hampton Court dates back to Henry VIII and is an easy daytrip from London, 1/2 hour by train, as is Windsor Castle.

Inside London there are all kinds of off the beaten path museums, the Museum of Brands, Packaging, and Advertising is one of my favorites, the The Wallace Collection, Kenwood House, Leighton House Museum and Sir John Soane's Museum are also lovely.

In addition to the daytrip options already listed there's Hever Castle & Gardens and Hatfield House and Warwick Castle

No matter how many times I visit London, I know I'll find something new on a West End theatre stage. A visit to tkts.co.uk and to timeout.com could help you find what's playing and how to save money on tickets. The Globe Theatre and National Theatre sites will also be helpful. 

Other reviewers offered excellent responses to your question. The Tate Modern regularly presents new exhibits.

If she's already seen all the major sights (e.g. Big Ben, the Tate, the British Museum, etc.), then maybe the focus could be on "experiencing" London rather than "seeing" it.  Perhaps going to Borough Market, going on a food or pub tour, or going on a high-end shopping spree at Harvey Nichols London or Fortnum & Mason, or perhaps exploring some of London's vibrant but less visited areas such as Brixton, Spitalfields, Southwark or Hackney.  If you and your aunt are into sport, then a football match is a great to experience London as the locals do.  Tickets to Premier League clubs such as Chelsea, Tottenham, Arsenal, Crystal Palace or West Ham are expensive and hard to come by, but lower level clubs such as Queens Park Rangers FC, Fulham Football Club Limited, Brentford Football Club or Millwall Football Club The Den have cheaper seats that are easier to get, cheaper beer and just as enjoyable an experience.  

I suggest:

1. An afternoon tea experience at The Ritz London or Claridge's.

2. Take a London football tour and visit one or two of the world's most iconic football clubs - Chelsea, West Ham United, Arsenal and Tottenham Hotspur.

3. Join London's Secret Food Tours to Borough Market (London's oldest market) - eat Bread Ahead's famous doughnuts.

4. Go to a neighborhood pub or coffee shop. Londonist has a list of the best coffee shops in London.

5. Take a walk in Shoreditch with Shoreditch Street Art Tours: https://www.shoreditchstreetarttours.co.uk/

I don't know what your/her tastes are, but generally I find that most people have not managed to make it to the Old Operating Theatre Museum & Herb Garrett. It's a little weird, but I found it fascinating. 

Also, maybe try the Museum of Brands, Packaging, and Advertising. It's definitely not one of the bigger tourist attractions. Has she seen the Design Museum? That's one of my favorites. Or how about the Fashion and Textile Museum? 

I hope that helps! 

I'd go with a stint in Shoreditch. This place rocks for a cocktail and a round of epic crazy golf Junkyard Golf Cluband if your aunt is one of the many who have watched Breaking Bad she would love this ABQ London.

On a recent visit, I went to the National Gallery, walked through China Town, explored Primrose Hill (great view of London skyline) and took a boat trip up to Hampton Court Palace (the boat also goes to Kew Gardens, which I'll do next time....)

I would take one of the many walking tours from Jack the Ripper to Abbey Road Musical Mystery Tour to whatever.  I have loved them all!

Drop Auntie off in a pub, and go see London! You haven't been, so see as much as you can. The The Tower Of London (Stop TL) is my personal favorite, but... Big Ben, Westminster Abbey, Houses Of Parliament Shop, a walk through Hyde Park... museums if it rains (most are free!), Speakers' Corner if you're there Sunday morning... the hop on- hop off buses will give you the lay of the land.