Honestly it's so easy to plan a trip there. You could do it using the resources on this board if you want to. If you have an idea of what you'd like to see and do, we can help.

I usually do all of the planning myself with the help of a good guide book, the internet and this forum. However, I have needed the help of a travel agent from time to time and have had good results with Pat Hewitt at Costa Rica Travel Exchange (www.lovecostarica.com). He is based in New Orleans but has a home in CR. He can set you up with hotels, shuttles, tours, etc.
Have fun! We just returned from our 3rd trip there.

Yes, Pat always gets great reviews on the boards.

He hired a driver who runs a tour company, with his wife, as well. His name is Wilson Rojas (wilson_elsy@yahoo.com), http://www.coatitours.com/. He ended up giving us all kinds of advice regarding hotels, side trips, restaurants, etc. He ended up arranging some of our excursions (sometimes getting a special rate) and also made connections for a private nature guide (in La Fortuna). He and his wife, Elsy, were a pleasure to deal with (all by email prior to the trip), and Wilson was a great driver, and a pleasure to spend time with (during our drives).

Based on our experience, I would definitely trust him to arrange a trip. I tend to do lots of research on my own, especially here on these forums, but did take much of his advice.

Do you mean Serendipity Adventures? I had never heard of them, but from the little bit of info I was able to find on the Net, they look like a good (but expensive)travel company that offers small customized tours. They are listed in 2009 National Geographic as one of the best adventure travel companies: http://adventure.nationalgeographic.com/2009/02/adventure-ratings-list-text

Also mentioned in Forbes: http://www.msnbc.msn.com/id/25266900/

A Lonely Planet review:
http://www.lonelyplanet.com/costa-rica/activities/other/serendipity-adventures

There are also a couple reviews on tripadvisor:
http://goo.gl/UBjh5
http://goo.gl/Ejasj

Serendipity is a good company, maybe a bit expensive. Here is a good cross section of companies I like here and why. I have used and or have had friends and family use each one. I like companies that are involved in charity and give back to the country in some way shape or form.

Horizontes – http://www.horizontes.com/eng/ - Very green company, offers packages, involved with community programs, good company & good folks
Crreferals – http://www.crreferrals.com/ - Good for custom planning, has packages, involved with animal rescue, good company & good folks
Actuar - http://www.actuarcostarica.com/app/cms/www/index.php Off the beaten path rural tourism, involved with community programs, good company, good folks.
Caravan – http://www.caravan.com/ - Best for larger group tours up to 30. Excellent reputation and well respected. Awesome at what they do.
Costa Rica Expeditions – http://www.costaricaexpeditions.com/wl3/index.php - High end trip planner, owns some of there own lodges, has won many awards, good reviews.

With a little time and hard work a trip to CR is totally doable without a tour company. Initially we were going to do a tour set up, but after researching and reading posts on here we undertook the trip ourselves. Don't get me wrong, it was a lot of work but so worth it. We were away for 2 weeks and saw Arenal/Monteverde/Manual Antonio and Drake Bay. It is all very tourist friendly and there was not even one hiccup our entire two weeks.

Use a Travel Agent! There is usually no fee for their service. I like Eco Adventures.