What to buy she who has everything?

Barack and Michelle Obama will join the Queen for her birthday. Photo / AP

The Queen will celebrate her 90th birthday dining with Barack Obama and enjoying a cake baked by the Great British Bake Off winner.

She is marking her actual birthday on April 21 with three days of events in Windsor, before more elaborate festivities in London next month and on her official birthday in June.

Obama will travel to Windsor Castle to dine with the Queen on Friday, in what is expected to be his last visit to Britain as president.

Commentators will be watching closely to see what the president gives as a birthday present.

Obama was criticised in 2009 for giving the Queen an iPod containing his own speeches and pictures from her state visit in 2007. His offerings improved in 2011 when he presented a set of leather framed pictures of George VI and the Queen Mother during a visit to the US in the 1930s.

Yesterday Great British Bake Off's Nadiya Hussain let slip that she is baking a special orange drizzle cake with orange curd and orange butter cream for the birthday lunch on Thursday at Windsor's Guildhall.

The birthday celebrations will begin on Wednesday when the Queen visits the Royal Mail delivery office in Windsor to mark the 500th anniversary of the postal service. She will also open a new bandstand at nearby Alexandra Gardens where local schoolchildren will perform songs and Shakespeare recitals.

The following day, the Queen will unveil a plaque marking the start of a 6.4km Queen's Walkway at the foot of Castle Hill in Windsor. It connects 63 points of interest in the town, commemorating the Queen's achievement in becoming the longest-reigning monarch after more than 63 years in 2015. The Queen and the Duke of Edinburgh will then go on a walkabout.

In London, Prime Minister David Cameron will lead MPs in paying a day of tributes. A 41-gun salute will take place in Hyde Park and a 62-gun salute at the Tower of London.

In the evening, the Queen, accompanied by the Duke of Edinburgh, Prince Charles and the Duchess of Cornwall, will light the first in a chain of 1000 beacons across the country and worldwide in celebration of her birthday.