Composting Toilet — there is a compact model made for boats, but a blackwater tank is required for excess urine, which can be emptied at any sani-dump station
Kitty Litter - Buy litter that comes in its own bucket. Use the kitty shovel to form a depression in the litter so your contribution doesn't touch the side walls of the bucket. Make your contribution. Use your kitty shovel (or an inside out zip lock style baggy over our hand like a glove)to cover it up. Either immediately lift it out or let it set up a bit. Remove it from the kitty litter bucket with your kitty shovel or inside out zip lock style baggy worn like a glove over your hand. Zip close the baggy and deposit it in a dumpster. Drive with the lid closed on the bucket and placed so sudden stops won't tip it over.
The simplest method is to lay out some newspapers. After defecating, wrap it in newspaper, then tie it up in a grocery bag. Dispose of it in the nearest garbage can or dumpster.
Newspaper method #2, wrap in newspaper with sawdust (to absorb moisture) and a handfull of soil, store outside where it can dry up w/o smelling, turns into a humanure compost burrito, yum!
Reclosable jug such as water or milk. Ladies use a funnel which is rinsed clean with water before storing.
Several plans exist on the internet for making a toilet out of a 5 gallon pail.
Gas station/fast food joints
Good for going to the bathroom, though frequently unclean and smelly
RV toilet/blackwater system
Composting toilet. It may be a little large, but there are [plans for a cheap 55 gallon drum composting toilet].