Shinrin National Park in Namegawa have a Bonsai exhibition this month. From the 3rd until the 25th, you can view Bonsai trees in the Botanical gardens of the park.
They also have a very special event for one day only, where you can pay to make your own Moss Jade bonsai. They will have two workshops on Saturday the 17th, the first is 10.30 to 12.00 and the afternoon session is 13.00 to 15.00. It costs 500 yen and they will accept up to 15 people for this rare opportunity. You also have to pay the entry fee to the park and parking costs. Reception is on that day, at the Botanical Garden centre, which is a 10 minute walk from the central car park.
Shinrin park is also known as Musashi Kyuryo National Government park. The official english webpage is: http://www.shinrinkoen.jp/english/