Goodness! I don't think that I can choose one, but here are some of my favourites over the years:
A four night stay at Champneys with £200 spending money
A two week holiday in Scotland, with vouchers for various activities
A weekend staying in a Hotel, with a hot air balloon flight thrown in
A spa day for two at Bath Thermae spa
Those are things that I would never have done for myself and then there are various large cash sums, which are always useful, laptops, tablets, cameras, various years supply of things, smartphones, TVs, games systems, to name but a few.