Now might be the time to convert your pets to the healthiest environment you could ever provide.
The most common statement we hear about why people continue to let their cats out is "they were born to be free". We've heard some very blunt professionals ask, "then why do you own them?". We'd rather remind the best intentioned owners that a natural environment for a wild cat (which should only be lions, tigers and their immediate relatives) does not include cars, mean-spirited people, dogs, anti-freeze and other poisons, threats or two and four legged predators. Also, catching rodents and lizards, toads and whatever - may be asking for major health problems, not just with the cat, but with diseases that could be spread to the humans in the house.
Disease such as Feline Leukemia and Feline AIDS wasn't nearly as rampant (or even in existence).
It was a different world when cats were born to be wild.
These days, if someone is going to own a cat, they must commit to being responsible companions. I assume you've already made sure they're 'fixed'. Neutering/spaying helps prevent quite a few diseases and disorders (including cancer) and studies in veterinary universities are repeatedly showing that the earlier they are done, the better their lifelong prospects.
Indoor activities that provide endless amusement for felines include sitting in front of a window that overlooks anything going on outside. Watching traffic or birds in a bird feeder, squirrels in a bird feeder too, people walking by or just the wind blowing a leaf and the sun moving across the sky - can be fascinating to a cat.
A couple of those little balls with bells are usually big hits, but so is a sock or thick rubber band (make sure your kitty isn't a rubber band eater - this wouldn't be good). For exercise, fill an old sock with a handful of dry beans (like kidney beans) or even rice - and sprinkle in some catnip. Then, knot up the end, tie a piece of twine securely around that knot and hang it from a door handle or just toss it on the floor.
Provide a proper scratching surface by just stapling a sample piece of carpet to a wall, a block of wood or post. Some people designate an old chair in a spare room for their cat's scratching pleasure.
If kitty begins to inappropriately scratch something, just quickly, quietly and gently pick her up, carry her to the appropriate place and put her two paws on it like she would hold them to scratch. Don't worry that she might not get it at first, she will. Eventually. Be persistent.
Ohio State University offers this http://vet.osu.edu/indoorcat
and another excellent link is
Indoor only will solve the problem, but you know, introducing yourself and kitty to the new tenants might also be a good idea, 'just in case'.