I'm not on the road yet but I have a lot of experience temping as an admin assistant. Basically, you will need to plan in advance and know where you will be for a while. Register with as many national agencies as possible and check in with them daily to let them know your availability. You'll need to go in to an office for an interview and take tests. Once you develop a rapport with a placement person in one office, they can probably contact other office locations for you. A recommendation goes a long way.
There are agencies that specialize in different kinds of jobs or specific industries, from administrative to "light industrial" to hospitality to promo/demonstration to helping set up and break down at conventions and conferences to accounting/bookkeeping to healthcare. If you have skills and experience in a niche market, that could put you in demand, depending on where you'll be.
As to length of assignments, it will depend on the client and region. It's been my observation that, for admin work, most companies won't need a temp for just one day if their assistant calls in sick, because of the expense - UNLESS there's something very important going on and they really need an extra body to handle an overflow of work. But assignments of a few days or a week or two is always a possibility. There could be reception positions for very short term because they might always need someone at the front desk. You just never know. Temping is unpredictable.
Short assignments do exist but the key is registering with multiple agencies and making sure you are ready to go in at a moment's notice. You'll have to do some research and strategize.