First ask yourself what is the objective of the report?
For you, your objective is to show that you are worth investing in and that you would value further opportunities to receive training.
Your boss arranged cover for your job that cost them time and company money
The other company facilitated you being there for 6 weeks they spent time (money) showing you the way they do business.
You were there to learn what exactly?
if you want to go on additional placements you need to prove that the 6 weeks provided value for money.
What is your current role?
Are you competent in carrying this out?
Why are you competent, was it internal training?, good processes?, transferable skills learned elsewhere?
How did you demonstrate that when you were in the other company?
What is the secondment role how did what you do day to day fit. As TeenDivided points out do your company do better, or where could they improve.
And remember both companies senior management could end up reading your report so give positive criticism with possible solutions where possible.
If nothing else you were given a valuable oppertunity to meet staff so a summary of staff you met and an overview their roles could be included.