Ask at any hotel or in town and this is one of the activities that everyone recommends. There are a couple of different options: one is to do a night hike in Monteverde Reserve itself, and the other is to do it through a little park in town. We choose to do it in town at the recommendation of the staff in our hotel, and we were not disappointed. They recommended it based on the time of day (the one in town starts right at dusk, while at the reserve it starts at 7pm); however, we were also happy to try a different location, as we were planning to spend a full day in the reserve.
You can sign up the day of in your hotel, and they send cabs to pick you up and drop you off. I believe we paid around US$15 for the tour, which was well worth it. Bring your camera! We saw tons of stuff.
We were divided up into groups (there were only two couples in our group!) and taken through the park to look for wildlife. We spotted LOTS of animals, which was great. Our very brave guide even coaxed several tarantulas out of their hiding places for us (much to the disgust of my traveling partner...)
Flashlights are provided, but if you have a headlamp, it's worth bringing. Also, it gets a little chilly in the evening, so bundle up.
The tour lasted around 2 hours and our guide was extremely knowledgeable and spoke flawless English (as did almost every guide we used for anything we did in Monteverde).
The hiking is not strenuous and is on well-maintained paths, so I would recommend this for anyone.
From journal One Week in Costa Rica