We've talked about buying a duplex and letting one of the buildings out already, but if you're not quite ready for that step yet, how about temporarily renting out one of the rooms you already own? You don't even have to do it long term if you're not comfortable; students frequently need short term accommodation. Especially foreign exchange or language students.
A student who's come to your area to learn English will probably stay for six weeks. A touring lecturer or professor may stay anything up to six months. They need somewhere to live for that duration and paying nightly rates for a hotel is the most expensive way of doing it. What they really need is someone like you to open a room out for them. If you're feeling industrious, offer cleaning, washing and breakfast services to them and bump up the price accordingly.