Jayanta,
1. Make some quick screencast tutorials about contributing to Bengali Wikmedia projects and play them in loop.
2. Try to hire some college students part time / unemployed graduates to man the stall. If this was not included in the grant, try to pool money from the community. Completely running the stall with volunteers for 12 full days to meet 1000s of people can be tiresome. 1 volunteer a day to oversee the hired people is practical.
3. Try to make some primary school level picture banners (on topics like animals, birds) printed in Bengali and put them for sale. This is something that you can ut together easily by collecting pictures from Commons. I have seen lot of such educational content in English but not in Indian languages. Ideal way would be to go to the book fair with an actual book compiled from Bengali Wikimedia projects. May be next year you can try it.