It may be a little ambitious to extend a list I haven't even started to tackle yet, but having discussed the existing list with friends and family, some awesome ideas for additions have been suggested:
Play hide and seek in a museum
Walk across the zebra crossing on Abbey Road
Sign my name on the wall of Abbey Road Studios
Go ice skating in Hyde Park
Pee in the one-way mirror toilet
A discussion about the list prompted Metal Dan to challenge me to pee in this...
Basically, it's a public toilet outside the Tate in London featuring one-way mirrors, so while you're having a little tinkle - or birthing a tapeworm, your choice - you can see everything that's going on outside. More info on it can be found HERE.
Sadly, after contacting the Tate I've discovered the toilet is no longer there, so I'm now on the hunt for a similar one. If you happen to stumble on one while emptying your bladder, please let me know!