Long slow distance work and hill work.
Hillwork! As well as trotting/cantering sets! They are super fun to do and will condition your horse really well!
First of all can you be more specific on what you call "older horses"?
For me, old horses should be on retirement on a field with lots of love, caresses and snacks!
But of course you can still ride him if your vet is ok and your horse still in good shape but only for walks on the woods (that will actually give him muscles and a great mental!).
Then, it depends of what the horse did before but you can also make some dressage but never go too far (do a to of full neck extension to stretch him), if he doesn't do it, don't insist, if it was a jumper you can keep jumping little fences to keep his mental but don't look for performances.
Make long warming-up to know what your horse can give you every days, listen carefully to him. Make him go to the paddock every day (with young horse it is perfect, it will become their "daddy" and teach them what they can't do, it will keep him busy and give him the will to live).
Finally, i would advice you to do lots of playing games with him (Parelli, playing with balls...) to keep having a better relationship with him without making him work too hard.
I hope i could help you :)