Closeups. With limitations like that, go for closeups on the face. Doing this you don't have to wait for sunset, you can do them indoors, using either flash or standing shop lights...even available light.
Pay close attention to your backgrounds, still. If you're using flash and a narrow aperture(higher number), your backgrounds will be visible and semi-crisp, so make sure what can be seen you WANT to be seen. You can always open your aperture(smaller number), which will allow more light into the camera and also blur your background, which is a nice look, and elimates *some* of the background clutter issues we all face.
The insides of barns and stables can make excellent photographic subjects, often times. Wood that has been brushed against by humans, horses, and other animals, over time, developes very warm hues and a nice gloss and can add tremendous depth and texture to the overall image. The tools that hang on the walls, benches, even dust and hay particles all create incredible ambience and can add loads of character to your shots. Stablehand feeding an apple wouldn't be a bad shot, either.
If I were you, I would delve deeply into the manual for the camera, and find out how to adjust your aperture and ISO. This way YOU can decide if you want long depth of field or narrow to either bring the background in focus or blur it out, instead of leaving it up to the camera. Than see if you can get permission to go to the stables on your day off and shoot. And just shoot literally
hundreds of pictures of all the horses, all the people, all the "goings on" within a day's activities in a barn. Bring as many memory cards as you can, and shoot as many pictures as they will hold. Hopefully you'll get 1 good one of each horse.
If you can put their horses in a photograph that would sell to the general public, you will find some
incredibly greatful owners. Plus...it's gonna cost money to get them pruinted, matted and framed, so you might as well make them photographs that anyone would be proud to display over their mantle in the living room. To give you a rough Idea, I pay about $70 to get an 11X16 print matted and framed to 18X20.
Anyhow...my point is that you have fantastic subjects in a potentially fantastic setting. It doesn't take a tremendous amount of skill to get really nice shots. 90% of it is being in the right place at the right time with camera in hand. Just a little bit of technical advice and creativity makes up the rest, and I'm happy to answer any questions, publicly or via PM, to help you out.
P.S.--remember to have fun