If you wanted to go the other way, from the National Portrait Gallery, you could walk West (about 20 mins) via Piccadilly Circus (do some sightseeing there), then go to Shepherd Market in Mayfair. It's a quaint little backstreet with pubs and restaurants etc which is absolutely charming and for some reason is lesser know and a little haven in that area. It's also close to Bond Street in case Mrs Mik wants to do some shopping

Or cut through Green Park, say hi to the King at Buckingham Palace and walk back along the Mall back to CG.
Also if you're doing art, the ICA is on the Mall