Garden Expo is running alongside in adjoining halls and can have some interesting ideas and sometimes seed companies exhibit so you can see what seeds will be available or even sometimes get some free samples. Tool Expo will also be there so always something interesting to see there and some exhibitors are selling their tools there. Often some good new tools you can try and in previous year there have been some expert craftsmen selling quality hand made tools.
To see a list of exhibitors and apply for tickets visit the website here Agri Expo Tokyo.
One thing I would say is that Makuhari Messe doesn't have enough spots for lunch so you either have a very limited choice or they are seriously over crowded so either bring your own lunch or eat before going inside.