Hi, and welcome back to Just Answer.
Trying to keep your cats inside as much as possible, is of course, the safest way to proceed. Erecting a very tall fence is another deterrent, but mountain lions have been known to jump up to 15 feet, vertically. Building an outdoor screened enclosure for your kitties would afford them some safety, but a mountain lion could most likely rip through the screening, if he wanted to. Mountain lions are most active at night, so allowing your cats out during daylight hours and watching them in your yard, would be a good compromise, but cutting down on their outside hours until they are indoor-kitties, only would be something to work toward.
Also, think about what attracts mountain lions; deer are food for them and if you have deer in your area as well, and/or if they come into your yard, think about what draws them there. Certain plants that deer will eat will attract them, and then mountain lions will follow, so try to avoid planting those types of plants. You can see more about ways to keep your kitties safe from mountain lions, here: http://www.mountainlion.org/4simplesteps.asp A motion activated sprinkler device called the 'Scarecrow' helps to deter stray cats and other critters from entering your yard, so maybe this would help with mountain lions, too.
some ideas for keeping them happier and enriching their environment inside, would be to set up window perches for them to look out and feel like they are still 'involved' with nature. Keep them busy and from getting bored with foraging toys, a large cat tree or kitty condo with different levels, 'hidey holes', and shelves for climbing, scratching, playing and snoozing.
Sorry, I forgot the link to the foraging toys: