The city is strollable, but big enough that you may want to take taxis, particularly at night. Cycling around León is not particularly pleasant, given the heat, the narrow streets and the traffic, but you can rent bikes for around US$5 to US$7 per day from hostels. Ruedas León rents scooters, motorbikes and mountain bikes.
Hostels are a good starting point for information. Quetzaltrekkers uses volunteer guides to lead volcano treks.León is also home to a range of grassroots, Nicaraguan-run NGOs. You’ll probably need a higher level of Spanish than if you were volunteering in Granada or San Juan del Sur, but chances are it will be more rewarding work. There are many NGOs at work in and around the city, including the Asociación Mary Barreda, which runs a variety of education programs focusing on women’s rights, sex education and STD prevention. Intermediate Spanish and a two-month commitment are required.