Just went to the David Bowie exhibit at the Victoria and Albert Museum this Sunday (among many other Stones fans) and if you're going to Hyde Park this weekend you should consider doing the same
The exhibition is really nice and informative. Both soundwise and visually: tv clips, wardrobe, handwritten lyrics, analysis of songwriting process and the usual trip down memory lane.
The thing is technically sold out but 450 tix is sold daily from the reception. Expect a two hr delay from the time you buy ticket untill your group can enter.

Nice little garden area with a pond and chairs and seats etc. in the centre of the building.

I just came back to Boston after a 5-night London trip - the 2 big reasons for going were The Stones at HyDE Park, and Bowie IS. Yes, the Bowie exhibit is fantastic. It IS sold out, but it's my understanding they make some tix available in the morning. It will also be coming to Toronto (Sept.?). A real treat for any Bowie fan.