goood things for cross training in winter?

i ride horses year round, but i ride less in the winter obviously. what are some good ideas for keeping in shape during the winter?

ive never been good at running, but i want to improve my cardio/stamina.

i want to keep my strong legs in shape- good things for that?

any good ab excercises? i want flat abs (im skinny, i just dont have that good of abs.)

thanks in advance!