I've got loads of things I could do with 400k, and regularly 'spend' more than that everytime I buy a lottery ticket. But the caveat that DH won't notice makes it a lot more difficult.
Even my DH would notice if we moved house, or suddenly sprouted an extension complete with a heated positive flow swimming pool.
I also presume we are not allowed to do sensible things like buying kids houses or investing wisely.
Replacing my jewellery and clothing with similar but higher quality would completely pass him by.
Sadly he'd notice a livery yard and a few horses, but a lovely bengal kitten could probably 'follow me home'. The garden would have loads of lovely plants that were inexplicably reduced to within my normal budget.
I'd rather shop in Waitrose, or Marks, but I could not deprive him of the joy of Aldi middle aisle.
OOOHH! I could 'win' a holiday home, in a competition I forgot I went in for. Perfect.