I suppose it depends what you mean by "historians" - if it's specifically the guys associated with universities and such, they write research papers, teach, grade assignment, write books for extra income, take part in the intellectual millieu by discussing the current topics (For instance, in Scandinavia there was a semi-recent renaissance in regards to researching and redefining crusades). Then there's the whole discussions about what "history" is as a soft science, or whatever the appropriate english term is, and how far history should emulate the natural sciences. Lots of things are being written about the field of history itself, and there is money to be made in such publications as well. So, lots of different things depending on what aspect of history you work with. One thing that'll be more popularised in is the use of history in the media, specifically in moies, so that's something people will start writing more about.

If you're talking about those with an education in history (I'm not american, so instead of the american system of majors and such it's somewhat different, so i do apologize if this doesn't apply to you!) they can also teach at what is equivalent to the american Highschool or work in a wide variety of jobs with elements of presentation or communication, like advisory positions and such. Some find work in the world of museums, of course. Others find work at archives or libraries, some work for the government with aiding in the identification and verification of historical documents. There are more possible jobs than most people assume.

I work for a number of historians in academia. As previously mentioned, a lot of an academics time, especially in term time, is to do with teaching and other committee work (for example, including mentoring junior faculty, strategy for the department, financial stuff). Another large chunk of what people I know do that might not be as relevant to some other historians, is working with the people and organisations whose history they study (but this is obviously limited to modern history really).

In terms of research, it's reading and writing and some primary research.

Primary research can involve interviews, consulting data (e.g. statistics) and archival work. Even if you'd think that history was just there, a lot of primary documents (letters, contracts, books written at the time, bills, shopping lists etc generic paperwork anyone accumulates essentially) are just stashed away somewhere. A lot of it isn't event indexed. So a historian will go to archives and look through it. You'd be surprised how much simple things like a shopping list can reveal about the economic status of the person who wrote it maybe or about dietary habits of the time or even, when comparing them and cross-referencing them to events, can show which events were deemed worthy of a better meal! Another thing is that a lot of things aren't in proper archives. Some records remain with the families for example. A fascinating example of a historian being given access to a families letters and documents is The Secret Rooms by Catherine Bailey in which she finds that what is especially revealing are the letters and documents that are missing.

A lot of time is also spend reading what others have written based on their research. And lastly, a historian writes an awful lot because what's the point of all that primary research if no one else will find out about it.