Anything can go into a writing journal because it is, quite simply, a collection of everything someone wants to write down. Especially pertinent for students, though, are responses to and questions about readings. Also, encourage students to think of a broad range of questions about what they read--questions about content, style, structure, audience, and so on.

Also students can use journals for other kinds of writing:

jump starters - snatches of conversation, radio/TV bits, billboards, songs, pictures--jot down anything that strikes you as an interesting image or idea
experiments--try writing about the same idea to several different audiences (Ranger Rick, National Wildlife) or in different genres; try out different analogies to explain a concept
record of observations--physical or mental
problem statement and problem solving
dialogues
process analysis
letters
interviews (including conferences with teachers and discussions with peers)
scenarios or cases (especially good for audience analysis)
reflections on writing process--questions/problems/successes