Lots and lots and lots of things. Carrots and spring mix aren't good for all the time. And even dandelion shouldn't be fed in very high amounts Variety is key! http://russiantortoise.net/care_sheet.htm Please read that! It'll tell you everything you need to know.
Also, @Tom has a great list of food that they can eat. It's so long, I can't remember what it says
I would mostly avoid carrots. Too high in sugar to be used as a regular food. Here is a list of 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