Our tour in Phnom Penh includes Silver Pagoda, Royal Palace, National Museum, Choeung Ek Killing Fields, Toul Sieng Museum and also the Russian and Central Markets. What else is not to be missed seeing for our free day in Phnom Penh before our flight out of Cambodia. Where else did you go that you found worth the time ? Our interests are quite varied, and will take any kind of suggestions. Please indicate how difficult or easy it is to reach the place (for example,is a car and driver required). Thanks so much.

That's a very full day. I'd have trouble getting through half those sights without running through them and not really taking the time to see anything. Frankly, rather than thinking about adding anything, I would think about what you could drop.

About the only think not in your list is Wat Phnom, but I personally don't think it's that big a deal.

I assume that you are asking what else is there to see on your free day after seeing all those other sites on your organized tour. That tour is covering a lot of ground and you may find that you want to see more of one or more of the sites that you were rushed through.

Other than those covered by the tour there are no realy other important must places to see. You could visit the Sorya Market, the first real shopping mall opened a few years ago.

Visit Wat Phnom as previous mentioned

Take a short river River Cruise that typically takes about 1-1/2 - 2 hours and is fairly interesting

Have a meal or at least a drink at the well known Foreign Corespodenc Clob (FCC) along the river.

Is the tour content fixed? You don't need a tour for the markets - much better to just wander around on your own. And I'd plan to go back to the Royal Palace to spend time with the Ramayan murals on the outer walls. Otherwise, enjoy walking the promenade by the river and chill out in the riverside cafes, feeding the street kids.

I think the tour that you suggest, it can almost one day. I think you can do to visit local market that you will see the different thing. You can also enjoy at The Riverfront, The evening stroll along the Tonle Sap riverfront is an institution among Phnom Penhois, particularly in the hot season when the cool evening breeze blows in over the water

So sorry for not being clear. We will be in Phnom Penh for 3 days. The sights I've listed will be covered in 2 days, so we have 1 extra day.
I came across something called Mekong Land (a showcase of Cambodian culture, food, etc). Also Udong, and other temples like Phnom Chisor, Tonle Bati/Taphrom Temple, and I was wondering if they are worth the trouble. We will have a car with driver on this day.
Thanks for your suggestions.

A good site to read up on the places of interest outside of PP is
http://www.canbypublications.com/phnompenh/ppnearby.htm
Actually this is a good site to read for PP and the rest of Cambodia.
I have never heard of Mekong Land.