Go west. If you're looking for amusement parks in the Tokyo area, the surviving ones seem to all be west of the city center. Some may be northwest like Toshimaen, others southwest like Yokohama CosmoWorld, but the east side just doesn't have much to offer. Sure, straight north is Tobu Zoo and Asakusa's Hanayashiki Park is on the eastern side of the city, but they have other attractions that draw the crowds (animals and history, respectively). That might be due to the existence of a certain mouse on the eastern side of Tokyo, though I'm sure weather and traffic patterns (including train lines) make a difference.
The water park was pretty crowded on a Sunday late afternoon but as I mentioned the rides part of the park wasn't. I could walk on to all the rides. Summerland felt almost abandoned, similar to many of the theme parks around the country. Several were built when times were good and money flowed like water from a hot spring, but financial issues have closed several parks and left most of the rest barely hanging on. Disney and Universal Studios certainly haven't helped things either. I really can't think of any truly new coasters in Japan outside of Disney, Universal, and Fuji Q.
A quick check of the Roller Coaster Database shows that no new coasters opened in Japan this year; two kids coasters and a replacement coaster (which is pretty good) for Joypolis opened last year, and only one new coaster is mentioned for next year, though it could be really fun.
Water Adventure Tokyo Summerland (even the water park comes first in the name now) is open from March through November with varied hours, and is closed one or two days a week in the spring and fall. Admission for adults starts at 2000 yen, with a free pass for the rides adding an additional 1000 yen; prices are higher in the summer when the outdoor water attractions are open. It's a 40 minute train ride from Shinjuku Station to Hajima Station, then another 10 minutes to Akigawa Station. Finally, a 10 minute bus will take you right to the park's front door.