I can't recommend this trip enough! There's no way I'd be able to see and truly enjoy this country without the help of Contiki. 8 cities in 10 days sounds overwhelming but it reached all of my expectations, and it was the perfect mix of adventurous/relaxing. All of our day/night tours (included and add-ons) were so great, and the people of Costa Rica were super friendly. I felt safe everywhere we went, even during free time.
Although I have SO many favorite parts about this trip, my top had to be waterfall rappelling in La Fortuna and visiting the national park beach in Manuel Antonio. We had such a great group (only 20 of us) and we all made it the best it could possibly be. If you're lucky enough to have Gustavo as your tour guide and Santos as your driver, you are in the best hands!
My only other call-outs (not Contiki related!) would be to consider the rainy/"low" season before booking. The week we went was the beginning of rainy season, so many popular areas like Tamarindo/Jaco were kind of dead and it down poured just about every day from noon into evening. Also beware of drinking tap water if you have a sensitive stomach! Many of us were not that concerned at the beginning of the trip, but all got a little sick along the way. A pro-tip I wish I knew beforehand was you definitely need a rain jacket handy, multiple sneakers (water shoes, casual dry shoes, hiking shoes) because you will be damp during many parts of the trip :)