IMO you can't really do much with a wet field, round pen, long dirt road, and trails for riding except having fun on trails. You could let him have a break from his training to have fun.
Ground work is something you could work on in the round pen.
Here are some things I like my horse to be able to do on the ground:
- stand square
- back up with light/no contact
- roll back on lunge
- ground ties (makes bathing a LOT easier)
And some things under saddle that you could work on when the field is dry:
- back up with light contact
- turn on haunches/forehand
- stopping as soon as you ask
- half pass (is that what you meant when you said leg yield?)
- extended & collected gaits
- & just to be fun in the round pen you could work on bridless
You could also teach him tricks, some really help stretch your horse like bowing.
During the winter is when I work on my horse's tricks, here are some that I do:
- give kisses
- shake hoof (can advance to count & spanish walk)
- yo-yo (Jessica Forsyth has a good video up of it; its a Parelli game)
Hope this helped. :)