three island crossing state park glenns ferry Idaho

It's about halfway between Twin Falls and Boise off I-84 at Glenns Ferry. Depending on where you are on 2, you probably will need to come down I-15 to I-84 and head west. If you do, then you will go through American Falls, which also has a very nice state park, Massacre Rocks. Incidentally, I think both of these parks give a senior discount on M-Th camping.