Writing-to-learn (WTL) activities take very little class time, and most teachers find they can give a quick WTL prompt at the beginning of class while they take roll and as students are settling in. Moreover, many WTL activities can be limited to just a minute or two—the amount of time it might take to answer a student's question about a course concept. Also, because WTL activities are such valuable learning tools, most teachers feel that students use any minutes given over to WTL writing very effectively.
If you decide to give a disciplinary writing assignment (a formal document that students work on over a long period and revise before submitting it for grading), then peer review is an excellent way to assure that students are revising. One way to save class time is to require students to do their peer review as homework outside of class. They will appreciate having class time for peer review, but you can either require that they meet to read, comment on, and discuss each other's draft or that they exchange drafts at the end of one class and return drafts and peer review sheets at the beginning of the next class.
Electronic tools now make out-of-class peer review much easier for students. Web 2.0 tools such as Google Docs allow multiple reviewers to embed comments in texts as they take shape over several drafts. Or if you prefer a more carefully guided peer-review approach, you could consider a tool such as Calibrated Peer Review (see Russell et al., 2004, or visit http://cpr.molsci.ucla.edu/Home.aspx for description and details) that allows the instructor to note the prompts for reviewing responses to a given assignment. SWoRD simulates a modified journal publication process as its framework for computer-mediated peer review (Cho & Schunn, 2007). Most classroom management software packages also include tools that students can use to exchange and comment on each others' drafts. Even if you want to take advantage of the lowliest high-tech tools, students can exchange drafts over email and embed responses to specific questions/prompts (also delivered electronically) through a comment feature in Word.
Cho, K., & Schunn, C. (2007). Scaffolded writing and rewriting in the disciplines: A web-based reciprocal peer-review system. Computers & Education, 48: 409-426.
Russell, A. A., Cunningham, S., & George, Y. S. (2004). Calibrated Peer Review: A writing and critical thinking instructional tool. In Invention and Impact: Building Excellence in Undergraduate Science, Technology, Engineering and Mathematics (STEM) Education. American Association for the Advancement of Science.