Just got back from a ten day trip in Nica. Did the Mombacho hike and canopy tour with Tierra Tours. Trapperchica is right--the hike is more like a walk. Very manageable. The trail is very well maintained and it's very accessible for non-hikers and those who aren't necessarily used to physical activity. We left for Mombacho at 9.00 in the morning and got back around 3.00. I don't know if you will have enought time after Mombacho to make a tour to Las Isletas.
I also did a moonlight hike of Cerro Negro with the Leon office of Tierra Tours. The hike is pretty managable but it was incredibly windy (40km or so). You have to carry your own board which did make it a little challenging. We started around 5.00 so we didn't have to deal with heat of the day. Cerro Negro was more difficult than Mombacho. I met some people who hiked Cerro Negro in the afternoon and said it wasn't too bad since they started around 3.30. Suggestion: don't try to stand when you surf down the volcano. It's much more fun and much less work sitting.
And I did the night tour of Masaya out of Managua my last day. You have the opportunity to see the lava and you also walk through some lava tunnels and hang out with the bats. Here's my pics of the trip: http://picasaweb.google.com/dputz1/Nicaragua2009#.
Cheers.
rafl--check with Paxeos and see what kind of rate they can give you to get from Managua to Leon.