Going Tokyo-Kyoto-Tokyo, plus a side trip or 2, or going to/from Narita by the N'Ex would pay for a 7 day pass.
But you can increase your savings more by expanding your plans and seeing some of the best sights in the area.
In Kyoto you could spend weeks there and not see everything. The three best sights though are the Kinkakuji Temple, Kiyomizudera Temple, and Fushimi Inari Shrine. Fushimi is the most time consuming; you could spend a couple of hours to over half a day if you want to romp over all the trails.
The Fushimi Inari Shrine is something people of all ages would love very much. And the Otagi Nenbutsuji is one of the most humorous and whimsical places you could visit in Japan - well worth visiting, even for kids.
Nara is often neglected by some tourists, which is a shame. Missing the Todaiji in Nara is like going to India and skipping the Taj Mahal. Nara Park has a lot of great places. A few other great places missed out even by those who go to Nara though are the Isuien and Yoshikien Gardens. They are gorgeous and not crowded at all.
If you are already seeing Tokyo, you can largely skip Osaka during the day. After the temples of Kyoto close down around 5PM, you can zip over to Osaka for the evening. At night the city comes alive and has some great places to see, such as Dotonbori, plus the night views from the Umeda Sky Bldg, and Abeno Harukas Bldg are wonderful.
It's also possible to go see Hiroshima/Miyajima.
If that is still not enough, there are many places between or close to Hiroshima. Just a bit past Miyajima is Iwakuni with its iconic historical bridge and mountaintop castle. Onomichi is famous for its temple tour and wildly ornate Kosanji Temple. Nearby Kurashiki is a famous own city for its canals, and Okayama also has its own castle and one of Japan's other Top 3 gardens, Korakuen. If your kids are itching to explore, then in Kagawa there is the Konpirasan Shrine in Kotohira, with a marvelous paved mountain hike.
