I don't know what your budget is, but remembering when my son was 2, I wanted my wife to have a break from children for the day. I arranged for someone to care for him and I took her on a day-trip on the Orient Express.
http://www.orient-express.com/web/bp/bp_a2a_ho me.jsp
There are late break rates available and the train departs from Victoria Station. There are alternative trips from other locations.
If this doesn't appeal, perhaps a 'pamper day.' I gave my wife a day at the Thalgo spa and I once gave her day at the spa at the Mandarin Oriental Hotel in London. I'm sure there are similar wherever you live.
Best of luck
Fr Bill