You will probably visit Rivas [1] on the way to and from San Juan del Sur [2]; half a day is acceptable to walk around the historical sites, appreciate the cathedral, and perhaps take in a museum. Travelers with a more leisurely schedule can easily spend another half day enjoying the lakeshore in San Jorge [3] before continuing to Ometepe.
La Isla de Ometepe [4] should not be missed on any but the shortest trips to Nicaragua, offering in a nutshell, a little of everything Nicaragua has to offer, from history to waterfalls and from volcanic trekking to horseback riding, all in an environment travelers routinely rave about as relaxing and delightful.
You could feasibly travel to and from the island in a single day, but such a short trip would be folly. Rather, allow at least two days and two nights (and an extra day and night if you’d like to hike a volcano, which is a full day activity in itself).
Note that travel in this region requires careful coordination of transport, as you can easily lose up to a half day waiting for boats and buses. Traveling around Ometepe is never easy; local transport is slow and erratic (especially on Sundays) and renting vehicles can be expensive.
Getting around the island is difficult, no matter how you slice it. You can take a slow, rickety, infrequent bus on the worst roads you’ve ever seen for under a buck, or you can shell out $25–35 for a taxi or microbus to take you to the sites. You can also rent a motorcycle or scooter, but aside from a few motorickshaws or scoring a ride by hitchhiking, there’s not much in between.
Once you’ve accommodated your stuff at your hotel, biking can be an enjoyable, cheap, and relatively fast way to get around the island if you take it easy, drink plenty of water, and don’t mind riding a clunker. If you’re traveling with a cell phone and speak Spanish, it can be helpful to get your taxi driver’s number so you can call later and make arrangements yourself.
Links:
[1] http://www.moon.com/destinations/nicaragua/ometepe-and-rivas/rivas
[2] http://www.moon.com/destinations/nicaragua/southwest-coast
[3] http://www.moon.com/destinations/nicaragua/ometepe-and-rivas/near-rivas/san-jorge
[4] http://www.moon.com/destinations/nicaragua/ometepe-and-rivas/la-isla-de-ometepe