I LOVE my Snugpax pommel bag:
http://store.runningbear.com/catalog/display.php?cartid=4efbc3906dffd&zid=1&lid=1&cat=30&scat=207
I really don’t need to take a heck of a lot more than this holds with me out on the trail. It fits two water bottles in the side pouches and the top pouch can hold smaller stuff like a leatherman type tool (I use the pliers part on there as a hoofpick), granola bar, a whistle/compass/thermometer thing, a glow stick, and a small Mag-Lite.
I also have a card pack that I put my ride card and trail map in that I clip over top of the other one. Its easy to pull off as we’re walking into camp so I can have the card ready for pulsing in and vetting in.
http://store.runningbear.com/catalog/display.php?cartid=4efbc3906dffd&zid=1&lid=1&cat=30&scat=208
I have a cantle bag, I believe by Dura-Tech that isn’t nearly so well made as the Snugpax. It gets the job done, but I like the Snugpax a LOT better.
http://store.runningbear.com/catalog/display.php?cartid=49222f560d263&zid=1&lid=1&cat=28&scat=201 Its similar to the English one on here. I’ll probably be replacing my other one with a Snugpax one. Though I don’t typically do much that requires two packs for one ride.
My trail bridle can double as a halter (just unclip the bit hangers), but if they wanted me to have another halter I’d just stuff my thin rope halter in there.
As for a lead rope, when I trail ride with one (we often ride from one end of a park to the other where there is a restaurant with hitching posts), I tie it around my horse’s neck and then clip the snap to one of the Ds on the front of the saddle. It stays out of the way and its there when I need it.