Gunma got a lot of things to see in the summer! My recommondations would be the Tambara Lavender fields and Ozegahara! Both very beutiful landscapes. Other activities in the area Gunma and Nagano would be rafting, and I know that Nagano has mountain biking and paragliding to, so I guess Gunma also have similar activities!
When I planned my Japan trip last year I consulted with a company called Huber. , they helped me a lot with my questions even though I ended up not using one of their guide tours. http://huber-japan.com/?jt
Hope you get a wonderful time!