It’s a bit out of the Kings Cross area but I can’t recommend highly enough the Henry Moore exhibition at Kew Gardens. About 30 of his sculptures are wonderfully scattered throughout the landscape around the marvellous glass houses. (see reviews here and here) As the grounds are huge, you don’t get that crowded out feeling you normally get at the mega-exhibits. And you can touch and hug the pieces. Utterly brilliant there probably won’t ever be another exhibition like this in London.
The forecast is for a dry weekend so give it a go. It costs about £12 for adults, under 17s free. You can get to Kew very easily from the Kings Cross area in about 35 minutes on the North London Line – pick it up at Caledonian Road and Barnsbury station by the Ferodo Bridge on the Cally Road (map here check timetable here – usually three an hour).