The most popular time to travel to Costa Rica for most is from December to May. A time reffered to as the "high season". This time is also Costa Rica's summertime.
For those traveling to Guanacaste, the weather is going to be dry, since local life zone is a tropical dry forest. Trees will be in bloom, but is also during the hottest time of the year (March).
Another aspect to consider when traveling is the large amount of tourist flocking to beaches and overcrowding the tours such as zip lines. It is not uncommon for people to be stuck waiting on a platform to continue enjoy their tour. The beaches are crowded as well.
Traveling during the low season can save you money and you'll have less crowds and better service since guides won't be so overwhelmed with so many clients. Pricing at hotels also tends to drop.
For those interested in activities, don't be mistaken by assuming that due to rain you will be limited to what you can do. Things are rarely called off on account of rain, unless there is a serious storm (rare). In fact, the rafting is much better due to the water flow, waterfalls are more impressive and after all... it is a rain forest. Rain will also make zip lines faster, enhance the forest aroma with flowers of all sorts and also cleans the rivers.
Next week we will discuss ideas in regards to travel during the low season.