The weather report is the same year round for Costa Rica: raining! But actual weather is not only rain! Most probably you will encounter afternoon showers, so plan your outdoor activities for mornings, and your transfers for midday.
The amount of rain and timing varies from place to place and is less predictable than it was in the past. May is not the rainiest month where you'll be. It's unlikely that it will rain for a complete day but the timing of the showers may vary.
Even when it's raining, as long as there's no thunder, many activities continue as scheduled. You can hike, zip line, go rafting, etc.. Make sure to bring clothes and shoes that are comfortable wet and dry quickly.
rain, rain, rain, do not believe the hype. Weather patterns here are too complex for accurate reporting. It can be pouring rain on one side of a mountain with sunny weather on the other. Grab a poncho and go!
Some rain here and there but likely in the afternoons or evenings, nothing that will slow you down. Remember, you are likely coming here to experience in part the Rain Forest, rain is kind of part of the deal. Most activities run rain or shine. Some tours are better with rain, white water rafting, my favorite activity, is enhanced by a good rain fall. IMO no worries.