Nampa Recreation Center
Wednesday nights, something like 6:30-9:00pm (7-9 is a safe bet) in the lap pool. I also think they might have a Saturday morning session (like 9-noon) But I would call the center to verify before making the drive out. 208. 468-5777
Pay entrance to the Recreation Center (maybe 6 bucks) and you get access to 3 basketball courts, a weight room, 6 racquetball courts, all the aerobic equipment you can imagine, a track, and something like five swimming pools, hot tub, sauna, steam room. The kayaking takes place in the lap pool and I'm often the only paddler there.
When it was built in like 1993, the Nampa Recreation Center was considered one of the best in the nation. It's still a great facility.