Keeping my horse fit, no indoor

Haven't been on here in a while but all I found when I searched was people asking about when it snows.
I live in a climate that gets very rainy and wet, but almost never freezes or snows during the winter.
Hopefully I will be getting an all weather outdoor put in this summer, but until then, I have a grass outdoor, no indoor, and no trailer to get to an indoor.
I read somewhere that you can tone the back muscles by pushing up under the horses belly to make them contract. (tried it on her and it works the way they said)
So my question is, are there other exercises I can do with the horse in the barn to keep her from loosing muscle tone at least?

I'm sure that works, there's also "throping" or something like that, where you smack the horse with a cloth and they learn to tense their muscles? I dunno, there was a crazy video on it, something that "used to be done."

Also, do lots of road work. Just being out and about will keep your horse fit, and the concussion of the pavement is great for their legs. You can trot on gravel shoulders.

Consider asking your farrier about putting some shoes/studs on, to prevent slipping, if your outdoor is truly a grass outdoor and not just a grass field, and you won't tear it up too much.

I just ride outside. I ride in the corn fields. I ride in the pastures. I ride on the roads. I just ride wherever I can.

I have no indoor at all. Nowhere flat to ride, either. Just hills, some more sloped than others. I am at the mercy of the weather, but I still manage to ride about four to six/seven times per week, although it's worse in the spring and sometimes I need to take a week off to let things dry up some. I ride in cornfields, trails, pastures, etc. I would ride on the roads too, if the traffic was not so heavy and I wasn't hemmed in by crazy neighbors (long story). I manage to do OK; there is a lot you can do on the trails - basically everything you can in an indoor and more if you get creative. The horse also learns not to support themselves on the rail, and you learn how to eyeball a nice circle ;) .