How old is he, senior is not enough to go on, a 15 year old may be considered senior but have no reason to slow down a 30 year old will need a quieter life, I had a 28 year old pony here that was still in full work doing everything he had just at a lower competitive level, he only retired aged 30 because he dropped weight when he got cushings so any underlying health issues are also a consideration in how much work can be done.
I would keep doing pretty much everything they always have but cut back on any jumping, change the expectations when schooling so they are not progressing but staying at a similar level with an emphasis on quality of the work and their suppleness, possibly do more hacking, turn out as much as possible and keep them reasonably fit all year round.
I agree with BP, when I was teaching in America one year, many of the horses were over 20, and they all learned to jump (first time with riders for most of them, they were cow horses!). They were already fit and active, we did it in a large open pasture, and didn't over do it.
They learned to do telegraph poles as a grid. Learned to stay relaxed and in rhythm. I don't suppose the 30cm poles were too much strain.
I would avoid arena work with an oldie that was not already doing that type of work. Tight, repetitive turns are hard on older joints.
Really all an oldie needs is a big pasture and pals to enjoy it with (along with fly protection and presuming the pasture is not too rich).
I take my old pony for in hand walks 2-3 times a week. We either walk round our fields or go down the road and back. Probably amounts to about a mile each walk. I can only just keep up with her, especially up the hills. I occasionally get a yawn from her on the way back but she then has plenty of time for a snooze in the field. She loves getting out and about a little. She's 42 with Cushings.
We do all sorts with our 29 year old cob. He had a very active ridden life as a PC competition pony and fabulous hunter. He retired from the hunting feild 2 seasons ago with athritis in his hocks. Last got to PC Nat Champs the year before. He is still ridden - about once every 10 days or so for a breif hack by a very lightweight adult. I also broke / re-broke him to drive. (He was allegedly broken to drive at 4 but so far as I know never did - but he took to it like a duck to water!) We longline - but he will not lunge as he is blind on one side and will not turn his blid eye to the handler. He will ride and lead so hacks out unmounted from time to time. And about 18 months ago we discovered in hand horse agilty. He absolutely adores it.