We added oven cleaning last year, one thing we have found where carpets tend to be seasonal and there is a definite decline in the winter months, ovens remain steady.
3 ovens a day is easily do able, we dont even use a dip tank, just a soak tray, boiling water and the cleaning solution (vanquish and astonish), and while some I have found myself running away screaming like a girl from, once you get into a routine, its not usually too bad. Failing that I send the wife to do them
Cross selling has already been mentioned, and often while you are there up to the waist in the oven the customer decides they then want a chat, mentioning you also do carpets has often led to them booking that as well.
Ditto EOT cleans, while many younger tenants will eventually run around with a bottle of cif, when it comes to carpets and ovens they haven't got
a clue the time, so we get calls to do both on the same day.