My kids are older now, but things we've done over the year that were really enjoyed were:
A trip to London - tube rides, Madame Tussaud's, London eye a walk by the Houses of Parliament and seeing Big Ben.
Theme parks - Alton towers, Chessington, LegoLand and Drayton manor.
Harry Potter Studios.
York - Jorvik Viking centre, looking around the city at the historical sites.
Whitby - the lovely shops and steps up to the abbey.
Chatsworth house, gardens, farm & playground.
Beamish museum
Symphony Hall, Birmingham to watch many cbso performances over the years. Also The Barbican and Royal Albert Hall.
Theatre to watch Matilda and wicked.
Centre for Alternative Technology
Eden project (this could have been better imo)