So I just bring a picnick for me, some blanket and the most necessary tools to put on a new shoe if he'd drop one or I hear that dreaded metallic noise of a lose shoe... (I check them before every ride but you never know)
A halter and a halterrope, rthe bridle saddle of course and a breast collar just because I like the looks :P
We have plenty of creeks and small streams where I ride, so I usually let him drink and splash in the water when we pass a place, and he's tied so that he can munch a little on grass and leaves when we rest.
If I went on a longer trip, over a day or more, I would bring one of those portble buckets with me for him, since it's always possible for me to go down to a creek in emergency or just because, but it might be very difficult for him if it's steep and so on. Or I might ride past a house where I can ask for water, most people think it's nice to see riders and break the daily routine :)
I'd make sure to bring hay or something tho, unless I had a plan on where to go and could place hay or have a pasture there. I can't tie him up in a way that lets him graze enough on a trail :)
And of course other stuff I might need, depending on season etc.
The camelbak is a great idea! I know a girl that has one. I carry two large bottles of water for my horse and myself - there are streams where we ride and the horses have opportunities to drink (luckily, we've had a lot of rain this past summer!). I carry my little ER pack for my horse - I have something for a hoof injury - a Probiotic and electrolyte paste - I pack a lunch/snacks for myself and I share with my horse! I do have my cell phone, but getting a signal can be a chore! 
