If you do all your work for each company while at each company's location, you will need contexts listed for that site--you will need to analyze the situation depending on whether you use their phone, their computer, supplies, etc. I would bet you would have certain days or people whom you have tasks with, so using the calendar and @adjenda would probably be crucial.
If you are like a lot of us, our work travels with us. It is done anywhwere that we can do it This is both good or bad, but your ability to track and manage your tasks and projects will be crucial. Also, developing a good relationship with a key person at each site might help-the day before you are due their you might contact each week or whatever pattern works for you. I had a partime job where the boss always put copies of anything I needed in my mail box and was really great. This spoiled me and in my next one, the boss was not so thoughtful, maybe even resentful rather than grateful to have me. I had to constantly be on the look out for changes in meeting times, places, policies, etc. It really was a burden. They didn't even let me know in advance that the water was going tob e off for a day or that the furniture was going to be "touched up".
Also allow time for any administrative business at the site such as forms, human resources stuff at each place.
Either way, having a reliable capture and review will be crucial. Good luck. You sound like an energetic and well-directed person, so once you think it through, it will all come together for you.
If you carry papers about, you should photocopy anything irreplaceable and keep a copy at the site it belongs to for safekeeping.
You may want to have a different colored folder for each site. Maybe a folder large enough to put smaller folders in if need be.