You are SO RIGHT to keep your babies inside! There are just too many ways for them to get sick or hurt if they go outdoors. Fleas and other parasites, communicable diseases they can catch from other cats, cruel children or evil people who might hurt them, cars of course, and other animals that could attack them, from tougher cats to dogs to all sorts of wildlife, even in the city.
I live in Dallas/Fort Worth, a huge metroplex, but there are raccoons, armadillos, rabbits, opossums, even coyotes... brown recluse spiders and black widows and rattlesnakes... I've seen all of these critters myself, and I would never put my sweethearts outside to get attacked or catch some horrible disease.
In this area, the SPCA says that the average life expectancy for an outdoor cat is only two years. I just won't let my babies run loose.
Forgive me if I repeat something someone else has already suggested, but here's what we do to keep our little ones entertained indoors:
~ Comfy padded shelves that attach to the windowsills so kitties can bask in the sun and watch the world go by. When weather permits, we open the windows just a couple of inches so the kitties can enjoy the outdoor scents, too. (Never any wider than that, though, lest the screen give way and a kitty go flying...or a passing dog come bounding through!)
~ Bird feeders! I use a variety of types with different seeds to attract as many different birds as possible. I also have a couple of birdhouses, but nobody's moved in yet...must be a buyer's market...
~ Cardboard boxes, every size and configuration, duct-taped together with holes between to make a maze for kitties to climb around in. They LOVE this! I use a screwdriver to make a few small holes here and there so I can slip treats in for them to find along the way. Old boxes wear out and get replaced with new ones...I turn the whole thing upside down...the paths get altered and the kitties get to discover it all over again.
~ Another box trick: You know those wonderful little lightweight foam balls (they look like little colored golf balls) that you get in packs of four at PetCo or PetSmart? I put a couple of those into a shoebox, duct-tape the lid on, and cut a few holes just large enough for kittypaws, but not large enough to let the balls roll out. The kitties bat the balls around in there, become obsessed with getting them out, and eventually destroy the box altogether. Sure, you can buy toys similar to that, but the box is free AND they can tear it to shreds! What could be more fun?
~ Finally, have you ever seen any of the cat videos that are available? We have just one, a couple of hours of closeups on birds and squirrels in the wild, fish in an aquarium, and mice in a little habitat. If you play it too often, they get bored, but a couple of times a week, it's great fun. (Our Dylan, one of the smartest cats I've ever seen, stared at the video long and hard, then climbed up into the TV cabinet and inspected the back of the set to see if he could get at the critters!)
Bless you for taking such good care of your little ones...good luck!