The Saturday Lack of Plans

I still can't decide what to do in and about London on Saturday.  Am accepting input on the following three selections.

Item One: Eurostar to Paris.
Advantages: Um, it's Paris. Never been.
Disadvantages: Very long day (starts at about 5:30 a.m., and I'm flying in the day before and taking a train out to a friend once I get in, so I will be well and truly wiped).  Expensive.  (Train itself is upwards of $300).  It's too late to book an escorted tour, so would have to make my way with a map and whatever French I remember from two semesters twenty years ago.  (Ou ce trouve le ... Eurostar?)

Item Two: Cheap tour to Canterbury and Leeds Castle.
Advantages: Never been. Probably cheapest choice of the three.  (About $80)  Am promised jousting at Leeds.  Potentially will get back to London in time for a show (although whether one can have dinner before is subject to debate).
Disadvantages: Have never really WANTED to go there. Still ambivalent. Supposed to be raining.

Item Three: Bounce down hill in big plastic ball. (Zorbing at Spheremania.)
Advantages: I like bouncing down hills in big plastic balls. Also, the whole process takes no more than four hours (including travel time out to the location of said big plastic ball), so would have time to do other things in London.
Disadvantages: Supposed to be raining.  Costs substantially more than I paid to bounce down a hill in a big plastic ball in New Zealand.  (About $80, plus travel costs.)

Thoughts? (I love travelling alone, but sometimes it's hard to make up your mind.)