Have you read the sticky notes on what to and what not to feed?
http://www.redearslider.com/forum/viewtopic.php?t=6
http://www.redearslider.com/forum/viewtopic.php?t=5
Oxalic acid is something to stay away from because it can block the absorption of calcium, so veggies high in that like spinach are not good to feed.
Anything with vitamin A is great to feed-- carrots etc.
Most water plants are great as well.
If you have any specific questions ask-- it's better to be safe than sorry.
And as for grass-- be careful of giving turtles anything from yards-- they could be coated with pesticides or other chemicals. I don't think grass is harmful, but I don't think it adds much nutrition either.