Upstream, the fifth level of the first Crash game, is a river level that is considered to be quite easy by many people. It has the least number of boxes of any Crash game, having only 14. The gem in the level is considered very easy, but first, you must acquire the orange gem, which isn't found until later in the game. Otherwise, this level is quite simple. You'll find Flying Fish here, as well as some Man-Eating Plants, which kill you, regardless of whether you have a mask or not.
As mentioned above, the gem is extremely simple to get, even though you need the orange gem to get it.
Picture gotten from Crashmania.net.