As you can't hike up the active Arenal Volcano, I recommend hiking up the inactive Cerro Chato volcano. It's not conical like Arenal. It's actually covered with lush rainforest and looks like a regular mountain but it has a crater with the green lagoon on the top.
We hiked there from the Observatory lodge to which we got by 4-wheel vehicle (a must if you drive) and we paid 6 bucks to enter their premises. However, the hike from there was great. It started with 30 min walk on the relatively flat dirt road with picturesque landscape all around. Farms, horses... You pretty much pass Arenal volcano and see the steam from its lava. Once you get to the bottom of the Cerro Chatto, a strenuous 90 min hike starts. It's a steep dirt trail that turns into mud when it rains. But the hike is well worth the view once you get to the top.
You can't see what's around the Cerro Chato as you climb up nor when you get to the top as the view is blocked by the rain forest. However, you'll get a wonderful view of the green lagoon that is inside of the crater on the top. You can also take additional 20 min to hike down to it and take a swim or camp. The hike down the volcano takes almost as long as the hike up. We took only few short breaks for sip of water and it took us 80 min to get to the bottom of the crater. You won't get exposed to direct sun while hiking in the rainforest but it's still very hot and humid.
1) Don't go if it rains heavily. The trail becomes muddy and slippery.
2) Bring at least 2 liters of water per person. Seriously!
3) Bring mosquito repellent and camera.
If you hike in the afternoon, you might want to end it sipping at the drink at the Observatory Lodge. It currently has the best view of the lava flow at sunset.