For travel my cat will be confined to a crate for up to 30 hours. I will be able to give him attention, food, and water throughout but likely not able to take him out. How should I handle his digestive/urinary needs? Do I include litter in the crate (though then he is surrounded by it)? Can he hold it that long? Include "potty pads"?
I have volunteered for a rescue that performs frequent cat transportation (and my youngest, Romeo, traveled to me in Maryland from Florida).
The rescue uses a large carrier and adds a small litter pan. The litter pans are usually labelled "kitten size" at the pet supply store and do not take up more than 1/2 of the carrier. The remaining space in the carrier should be large enough for your cat to stand up, turn around, and lay down comfortably without having to sit in the litter pan.
The litter pan is secured in the back of the carrier (usually with duct tape) so it doesn't move around. There is enough litter to soak up at least one pee, but it is not as deep as you would normally fill a box.
I also suggest making sure you have some resealable plastic bags so that when the cat uses the pan, you can pull over and clean it. Neither of you will be happy for long with a dirty litter pan.
To clean the pan, first make sure the doors and windows to the car are all closed (and all travel companions know what you're doing so they don't open the door/window). You'll have to open the carrier and you don't want your cat to get lost! Then turn a plastic bag inside out over your hand and pick up the waste. Flip the bag back rightside out over the waste and seal it up. You can add more litter if needed.
Some cats get nervous when traveling and will prefer to sit in the litter pan. If you think your cat is likely to do this, you can add bedding that smells like home to the non-litter side, but otherwise it is a common thing and as long as you are able to promptly clean any waste it is not something to be overly concerned about.