April is one of the best times to travel to Costa Rica. This is during the peak of the high, dry season, just before the wet, low season is getting started. This means a majority of your trip will be rain free. However, do expect some minor showers in the evening that only last a few minutes.
Instead of needing umbrellas, you will need lots of sunscreen. The days last almost 12 hours and give out lasting sunlight for that whole time. Areas such as the beaches have little shade, so make use of an umbrella to cover up when the sun is just too much.
Temperatures hold steady in the high 80s and low 90s all month near the coast. If you visit the higher elevations and the forests, the temperature is about 10 to 15 degrees cooler on average. This is great weather for all the activities you may have planned. Do expect that most attractions are crowded during the month of April.
During May, the whole country transitions into the rainy season. However, the first few weeks of May will be very dry. In late May, the seasonal rains come. This means that your last chance to book travel before the rainy season is the second to third week of May. If you are planning a trip last minute, make sure to check the weather predictions. Do realize, though, that the weather could turn at any minute.