Have a look at the main Shelled section and there you will find a list of weeds suitable. Obviously with the weather as it is at the moment most weeds that are about at the moment are under several inches of snow. You can buy weed seeds to grown your own.
Florette salad bags can be fed, but avoid Iceburg lettuce, also the cabbage family and fruit.
What are you keeping the tortoise in and what temperatures have you got - ie a hot end/cool end? What sort of substrate? Presumably the tortoise was not hibernated this year?
One last question, do you have paperwork for the tortoise or a letter confirming that the tortoise was given to you as a gift/similar?
If you can post some pictures of the tortoise and setup others will be about later to help.