Sand can increase the risk of colic. Personally I wouldn't use it. As for gravel I used some myself around the barn, but eventually it all disappeared and I just let it be. I did read that walking horse on gravel for some time every day indeed strengthen the hoofs. However I'm not positive how good it is if they'll run on it.
We have heavy mats around the hay rack that helps us to keep that area dry.
This is from the person I know (from my local forum):
Well, I'll tell you what I did and my "lot" actually is dry 90% of the season...
Put down a good base of No. 1 or No. 2 stone, cover with a landscaper's cloth and then pack down about 6" of stone dust on top. For my run-in, I added an interlocking rubber mat floor and put wood boards at the base of my entire fence with a couple drains to keep my stone dust from washing out in the rain. The only mud I have is right at the paddock gate, and that's only because the contractor that I hired didn't account for drainage there - and even that is minor.
Although the paddock can become slushy, it has never gotten deeper than hoof-deep. It makes it very easy to clean, and if you keep it raked will not become mushy at all, the rain either runs off or drains out. Even in the worst of rainy weeks, my paddock has nice footing, the worst coming after a snow - but usually within 3 days of a week-long rain it's dry and back to nice again.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."