Lake Natomas is on the American River, not far below Folsom Dam. Kayaking is a big part of the activities going on here. It can get crowded on warm summer weekends, but it is easy to get away if you want to. It is part lake, part river, depending how far up you paddle. But don't paddle past the sign forbidding boats beyond this point. If you do, you take your chances with the correctional officer in the guard tower at Folsom Prison. A good place to sing a little Johnny Cash, though.
It is a pleasant place to paddle, lot's of birds and critters to see and an easy getaway for folks in the Sacramento area. The river/lake is nice and wide until it narrows down in the canyon. There are many little side sloughs for quiet contemplation and bird watching. There are bike trails on both sides of the river, accessible from county and state parks. I am not sure the length of the lake (6 miles?), but it takes 2-3 hours to go half way up (or down) and return. But I usually take side trips so I imagine the hardcore paddlers would make it much faster.
Other activities include rowing (sculling), sailing, fishing, and some strange water bike thing that CSUS Aquatic Center rents out. A quick note about the Aquatic Center, its a great place to rent a kayak (or sailboat, water bike, windsufer), and they have classes for most of these. And the CSUS sculling teams work out here, and hold sculling meets so you can watch that too.
Things to watch out for: There are rocks where you least suspect it. The water level can change due to upper dam releases. Also, this is near a metro area so be careful when kayaking alone.
For Nimbus Dam (Nimbus Flat State Park). From Sacramento, drive east to Hazel. Its immediately on the east side of Hazel.
For Negro Bar. From Sacramento on Highway 50, take the Folsom Rd exit at the city of Folsom. After the bridge over the river, turn left onto Greenback. The park is immediately on the left. From I80, take Greenback almost to Folsom and turn right into the park
There are few places to launch. On the Nimbus Dam side, there is Nimbus Flat state park at Hazel and Highway 50 (or, to avoid paying the fees, I think the CSUS aquatic center will let you carry your boat from their parking lot). The state park is great, you can park very close to the water, or there is a ramp. There is also a nice grassy area and a sandy beach and bathrooms with running water.
Near Folsom there is Negro Bar State park. You have to park up a hill from the launch ramp and the port o potty isn't so nice. Or you can use the park, nice and shady, grass, and sandy beach, and running water. But you have to carry your boat a ways. We usually launch at the ramp and paddle to use the facilities. Or there is a floating port o potty halfway between Negro Bar and Nimbus Dam.
http://search.parks.ca.gov for state parks