Broadleaf weeds (of the right kind), certain leaves, certain flowers, squash plant leaves, spineless opuntia cactus once in a while, round it all out with some mazuri...
Here is a care sheet with a section on diet: http://www.tortoiseforum.org/threads/russian-tortoise-care-sheet.80698/
Here are some more specific suggestions:
Mulberry leaves
Grape vine leaves
Hibiscus leaves
African hibiscus leaves
Blue hibiscus leaves
Rose of Sharon leaves
Rose leaves
Geraniums
Gazanias
Lavatera
Pansies
Petunias
Hostas
Honeysuckle
Cape honeysuckle
Leaves and blooms from any squash plant, like pumpkin, cucumber, summer squash, etc...
Young spineless opuntia cactus pads