Here is a plan for four days.
DAY 1 1. The Kremlin and Armory (2.5 hours). Open every day except Thursday. You will need a separate ticket for the Armory Chamber, buy it in the same ticket office. Keep in mind the Armory entering time: 10 am, 12 pm, 2.30, 4.30 pm and schedule your visit accordingly. Usually one hour is enough to walk around the Kremlin territory and visit its churches (keep the ticket for it). About 1.5 hours should be allocated to the Armory. There is a free audio guide in the museum, which can be useful if you are not accompanied by a tour guide. It may be smart to buy the tickets online in advance. Be ready to get an actual paper ticket in their ticket office showing your ID (the same you used when were paying online).
2. Manezh Square, Red Square, GUM, St Basil's, Zaryadye Park and Varvarka street (60-90 min). Climb a "tundra" hill in Zaryadye - the view overs the Kremlin and St Basil's are spectacular.
DAY 2 If the weather is friendly, get out of the city center to enjoy Kolomenskoye and Tsaritsyno estates - they are easily accessible by metro and have both natural and architectural sights to enjoy.
There is also an indoors option for this day - the Tretyakov Gallery. A good guide can bring a lot more to the visit, but if your friends are experienced museum goers they can be good on their own. Note that the Gallery is closed on Monday.
DAY 3 1. Christ the Savior Cathedral (20 min) history is a great illustration of Christianity adventures in Russia. Try to find a modern icon where crimes of bolsheviks against the Church are shown. Note that on Monday it is open only since 1 pm. 2. Muzeon Park (1 hour) is the place where the old Soviet statues were moved to after 1991. It is easy to walk there from the Cathedral across the bridge. On the way you can be entertained by a view of the gigantic Peter the Great statue over the river and maybe stop at Strelka Bar on Red October island.
3. New Tretyakov Gallery (1-2 hours) is hidden in a long grey modernistic building in Muzeon Park. Go there for Russian avantgarde and socialistic realism. Now they also run an exhibition of Yuri Pimenov whose vision of 1960ss Moscow is lighthearted and romatic. 4. Continue walking along the embankment to Gorky park and choose a nice place for lunch. OR, for more views of the Kremlin, walk along the river in the other direction - to Sofiyskaya embankment where special observation platforms above water have been built this year.
DAY 4 1. VDNKh (90-120 min) is an impressive architectural sight of Soviet era. Your guests can walk around elaborate pavilions of Soviet republics, have a scooter ride and take rest next to the fountain.
2. Worker and Farmer monument (20 min) can be seen from the upper level of Space Museum, and it is worth a closer look. The 25 meters steel monument is a symbol of Soviet Union, created by a female sculpture Vera Mukhina for the World Expo of 1939.
3. Space Museum (60-90 min) tells about Russian side in the Space Race - you will see Sputnik (not the vaccine, a satellite) , Belka and Strelka dogs, 1st generation spaceship and walk into a copy of MIR space station. There are English captions and audio guides. Note that Russian issued QR-code is needed for this museum!
Contact me if you need an experienced guide for your visitors!