If you've got a bit of a green thumb, check out the Kurume Ueki Festival in Fukuoka. The event will include a number of local vendors gathering together to exhibit and sell 100,000 plants in approximately 1000 types, ensuring there's something to pique the interest of just about any horticultural aficionado.
Along with the huge variety of plants and saplings available for purchase, there will be other fun activities like a moss ball making experience (a fee of 1200 JPY applies).
There is no admission fee to enjoy the atmosphere of the event, but do bring yen if you plan on making some plant purchases.