Does anyone have any interesting ideas for exercising dogs when it is too hot for them to run around outside?

Play in the sprinkler!

Playing tug-of-war can use a lot of energy in a small space, and you can do it in the air conditioned house.

Sign up for agility. They would have the agility training inside where its cooler.

outside play in a cool paddle pool

Firstly, how is 30 degrees Celsius hot? That's AMAZING temperature to exercise your dog in! I have a LONG HAIRED black Labrador/GSD mix that runs around joyfully in 110 degrees Fahrenheit weather. She has access to fresh cool water and can come inside any time she wants.

Secondly, if she's that old and THAT is somehow hot(!?) she should be taking naps more frequently.

All dogs require, daily walk/elimination times outside, 3-4 times daily 30-40 minutes each time. Walking is not exercise, They also require daily aerobic exercise, such as running/swimming and mental stimulation, such as obedience training, use common sense, if it's too hot for your chances are it's too hot for them, refrain from such activity, or do it only during the coolest part of the day, in limited amounts, dawn/dusk, on dirt/grass in the shade, not on pavement/asphalt.

get up very early in the morning. here in Texas, it doesn't get riproaring hot until about 10 AM.

Rinse her down with the garden hose or in the shower/bathtub and as long as her fur is wet she’ll be fine to play outside supervised. Her wet fur will keep her cool even extreme weather. The hot sun will dry he rquickly so just hose her down again or bring her in when she starts to dry. If you see signs of stress or her being overly warm bring her in immediately of course. My Labrador and I used to go for long walks in the wilder parts of town during 100+ degrees Fahrenheit weather in the middle of the day all the time. I hosed is both down before leaving and then we’d rinse down at neighbors hoses or in sprinklers along the way. We never felt to warm and never got heat stroke so long as we were wet. (We didn’t walk on pavement unless she was wearing boots and I always walked barefoot so I knew instantly if the ground was too hot.)

My dogs just seem to sleep the heat away, They crash on the floor & just sleep & sleep & only go out in early morning & after the sun goes down.

Don't worry about exercise in the heat of the day, no body wants to work out in this heat & neither should you dog. For right now you are doing good & forget the day exercises until the weather cools down. In the wild animals find shade & sleep the day away. They do not do much but lay around cause it is too hot to do anything.

Mental exercise can substitute temporarily. Trying to do some training indoors would benefit the dog.