A trip to Red Square is a must - also St. Basil's cathedral - and the Kremlin alongside. Inside the Kremlin try to get to the Armoury. Plenty of English speaking guides outside who will gladly give a tour.
The Pushkin Museum of Art is also worth seeing - beware as a lot of museums close on Mondays.
If you can get tickets to a ballet in the Bolshoi theatre - it is worth seeing. Tickets are sold at kiosks at most of the Metro stations - and in the ticket office near the Bolshoi. Tickets for theatre, circus, etc are all vey reasonable compared to Western Europe. Circus is good fun - but you might feel a little strange there without kids!!!
There is a market each weekend at Ismailova (metro I think is called Ismailova Park) where you can get lots of Russian souvenirs & some art (- & can barter). I think it closes around 3 / 3.30.
I liked to visit Kolomenskoye Estate & Museum - but it is really just a big park near the river with a few nice walks & towers. On the metro line also.
The metro stations themselves are all quite beautiful inside - worth looking at a few.
There are a lot of Western style restuarants etc. now so food won't be an issue. These used to be a very popular Sunday brunch at the Vienna Cafe in the Renaissance hotel. It is a few years since I lived in Moscow - so not sure if this is still in operation.
Enjoy the trip
It is a wonderful city!!
valc