Another activity gift, theoretically free but realistically a blank cheque, is a day with a personal shopper. While many big stores such as Selfridges, Liberty and House of Fraser don't charge for this, you may want to allow a large budget for all the new outfits that will undoubtedly become essential by the end of the day.
If you are quick to order online, you can go for gadgets. That essential site iwantoneofthose.com has all kinds of fun stuff including chocolate fountains at £49.95 (right) or even an L39 Russian jet (a bit pricey at £199,999). My sons reliably inform me that the Rhythm Sticks are great fun at only £9.95, although you might need some earplugs to go with them.
I also found some ideas on lastminute.com. Romantics can name a star for their loved one at only £20.They also do some more masculine items, such as football club shares (certain clubs only) at £29.95 or a personalised football book, reliving the great moments of your team for £40. Again, not all teams are available. My personal favourites are the cuddly Giant Microbes. For only £5.95, you can have a 15cm fluffy version of your favourite bug such as streptococcus or flu.