You can't visit La Fortuna, the city at the foot of Mt. Arenal, an active Volcano without going up to see the volcano itself. Arenal isn't just called "active", it actually has lava rolling down the side of it!
To find a guide, my wife and I walked down the busy tourist trap main st. in La Fortuna and signed up with the sketchiest pseudo-professional "tour guide" who approached us. Sketchyness is calculated as butchered english phrases x $ discounted during today's "special" / teeth. Our new "friend" said he'd even pick us up (right before sundown) at our hotel - we were sold.
Overall, things went as advertised and it ended up being on of the highlights of our trip to Costa Rica. Armed with only flashlights, we were led through the jungle and up a rocky mountain side in pitch dark conditions. After a half hour walk or so, we made it to the 1992 lava flow area. Turns out that 1992 was the last time this volcano erupted and lava flowed down to where we were standing - scary stuff.
Our tour guide finally found his favorite set of craggy rocks for us to sit on and watch the lava flow. It was fascinating. While the lava was understandably very far away and there wasn't much of it to be seen that night, just hearing the rumbling of the volcano and seeing some red/yellow/orange roll down the side of the mountain was an experience unlike anything I've ever seen. Its a must see if you're in Costa Rica.