Hi, JoAnn, I live in Prague, and while I have never used the Cook and Meet company, many of my friends have and say their tours are absolutely wonderful. Daniele Toniolo is the guide they have used, and here's their website: https://www.cook-meet.com.
For a can't-be-missed experience, it depends on what you're most interested in. I'm assuming the RS tour will take you to the main tourist sites, so I won't mention them. Here are three of my favorite off-the-beaten-bath activities:
1. If you're interested in art and history, I strongly recommend the Lobkowicz Palace. It's up at the Castle, but because it's a separate admission, it's never crowded. The audio tour gives you a capsule history of the region, as it was experienced by one noble family. The palace also has a lovely, reasonably priced cafe.
2. If you're interested in nature, I'd recommend a brief hike in Divoka Sarka, a stunning nature reserve with a rocky gorge, stream, and old forests. Take the 26 tram (from Namesti Republiky) or the 20 tram (from Malostranska) all the way to the end (it's about a 25-minute ride with interesting views of the city), and walk toward the McDonald's. Take the right turn about 100 meters beyond the McDonald's and then follow the switchbacks down to the gorge. The walk through the gorge is about 1 kilometer, at the end of which is a lovely beer garden. You can continue on the path along the stream as long as you like and then turn back to catch the 20 or 26 tram back to the center. This is a true Czech experience!
3. If you're interested in music, all the major churches offer tourist concerts. The concerts are of the "greatest hits" variety, but the musicians are truly outstanding and will give you a wonderful musical and architectural experience. I recommend either St. Nicholas Church (there's one in Old Town Square and one at Malostranske Namesti), or Obecni Dum (called Municipal House in English--it's near Namesti Republiky). You don't need to reserve these tickets in advance--they're always available at the door.
I hope this helps! Prague is a truly magical place. Oh: I almost forgot my first piece of advice. Don't forget to look up, everywhere you go. All the buildings have lovely frescoes, sgraffito, or ornaments. Have a great trip!
