Play games with the cat using its favorite ball or catnip toy. Cats also love those short wire poles with twirl toys or feathers on the ends. Some cats also seem to enjoy videos featuring sights and sounds of birds, fish, toys, or other cats. A carpeted kitty condo, scratching post, or climbing pole may provide stimulation. A perch can be as modest as a square piece of plywood covered with carpeting.
Brackets attach the perch to the window sill.
A variety of commercially-made products are also available. Indoor cats can get pretty irritated with outdoor cats. Abnormal behavior ranging from loud crying to destructive drape shredding can result. Try to shield the indoor cats from the outdoors. Cover your windows.