In Granada (and Rivas, too), there is a small immigration office. In Granada it’s located by the old train station & Parque Sandino. Excellent choice for those without a car or a day to spend in MGA.
It’s really easy: bring your passport, 2 photocopies of the photo page and the most recent stamp of your passport, and cash (same fees apply as above). You can renew for up to 90 more days. A little paperwork, and this small office will take your passport and mail it up and back to MGA. 7 days later, passport is back and ready!
In Rivas, the building is one block towards the lake and one block south from Restaurant Parilla.
I’ve done it many times and have had no problems!