Your itinerary looks very nice, but yes I would definitely suggest you to go to Ometepe! The island is an extraordinary sight as it is entirely formed by two volcanoes – one dormant (Maderas, 1,400m), one active (Concepcion, 1,600m). The views are beautiful as the twin cones grow ever larger until you disembark at the foot of Concepcion. The island is home to the Nahuatl people and has a long and fairly mysterious history – the isolated east coast is home to caves full of paintings and petroglyphs.
If you choose to hike to the crater of Maderas is a long, hard uphill route which is usually muddy. The forest is home to many monkeys, and abundant flora however, so there’s plenty to see along the way. The views from the top make the exertion well worthwhile. As long you are a fit walker, you should try this hike!
You can also walk to San Ramon, to see a beautiful 56m waterfall on the southern slopes of Maderas. The walk up only takes about an hour but it is a steady and steep uphill - birdlife is excellent!
For a more relaxing experience, take a dip in the volcanic springs at Ojo de Agua.

Another great option would be to visit Solentiname, is renowned for its distinctive naïf art, notably the bright watercolours featuring the islands, their flora and fauna. You could pick up a lovely painting here at a fraction of the cost they are found elsewhere. Within easy reach are islands full of howler monkeys, and El Zapote, also known as Bird Island for obvious reasons. While you can't land on these islands, hiring a boat is inexpensive, and gives you plenty of opportunity to get close to shore for photos. For more wildlife, Solentiname is a good starting point for a trip into the Guatuzos wildlife refuge. This beautiful area is home to three types of monkey, caimans, turtles and many birds. The narrow river makes it very easy to get really near to the fauna!

Hope this help!

Laurent