Were you thinking in UK or abroad. Paris is always worth a thought, close enough for one or two nights.
Uk, you always take a chance on the weather whatever the time of year and I would choose a place that has places of interest for her, eg Lake District for scenery, Canterbury for the Cathedral, Kew for the gardens etc
I was gonna say the Lakes but personally I would love to go back to Amsterdam (again and again) -it really is beautiful and warm at that time of the year.Lots to see and do and not a long flight.
I'd agree with Drisgirl... Amsterdam is a lovely city - you don't even have to go anywhere near the *ahem* coffee shops for fun ! The bars stay open until the last one leaves - it is a seriously chilled out city with very little nasty traffic - buy a bus pass and you are well away !
nokno -bi-curious and cant do space cakes cos i whitie -hope this answers your question :)
Jillius -Ive found there areas in every city we've gone Copenhagen Milan Barcelona Hamburg - but in Amsterdam Chinatown is the only pace I would really avid day and night -its very well policed so strangely its one of the cities i've felt safest in.That apart -the canals mueums the floating flower market just the atmosphere -its wonderful.
Took my wife to Barcelona for her 40th, can recommend Hotel Jazz. She took me to Rome for mine, but I much preferred Barcelona. Aren't budget airlines great?
If you want to stay in the UK then the Swan Hotel at Newby Bridge in the South Lake District is lovely - has a health spa and very romantic setting next to the river.