This was your original itinerary:
Day:
1. Arrive MGA and fly to Big Corn-->Little Corn
2. Little Corn
3. Little Corn
4. Little Corn-->Big Corn->MGA (early flight)->Leon (hopefully by 1-2pm)//Explore Leon
5. Leon--Telica @night
6. Leon-->Jiquilillo a at night
7. Jiquilillo
8. Jiquilillo
9. Jiquilillo to (learn to) Surf Camp then back home 8 days later.
You were thinking of skipping Jiquilillo (as you already have a lot of beach time) and going to Ometepe instead but you were hoping to have 1-2 extra days. If you don't have those extra days it will be hard to fit Ometepe in. And you also have only two full days in Leon in which you now also want to go to Juan Venado in Las Penitas (a 30-45 minute bus ride from Leon).
For nature and wildlife I would choose Ometepe but I don't know if you have the time. The volcanoes on Ometepe are great but not moderate. Both are hard climbs.
Esteli is easier from Leon and near Esteli there is El Tisey, Miraflor and the Somoto Canyon. All can be great but all take time to get to. As for wildlife... in that area it is mostly birds.