Prelims stress at its peak...

In my experience, the lack of confidence regarding preparation causes a lot of stress. I can suggest you to do yoga/meditation/sports etc. but this would not really help in the shorter time frame. Rather, I'd suggest to do short sessions of solving topic wise questions (say 30-40 questions of geography on day 1, history on day 2 etc. etc.) which will bring out the main issue- whether you need to focus on covering the syllabus or it is an issue of retention or it is an issue of not fully understanding the question etc. etc. This worked for me and my confidence would be shaken after every such test, until it helped me to prepare better. The stress of "not knowing enough" would eventually subside. That is my two cents.