I find the easiest way to get an object back into 2.79 is to append the object. After starting 2.79 (a new file not after opening the 2.80 file), choose File->Append and select the object from the 2.80 file, this brings in the object, modifiers, mesh data, materials and animation.
Directly opening a 2.80 file with 2.79 fails to read object data, you will find the mesh, materials and action data is there.
After opening a 2.80 file in 2.79, add a new object, in the mesh properties change its mesh data to use the original mesh data. Materials are usually associated with the mesh, but if not you can find the material in the materials list. If you go to the action editor (mode of the dopesheet) you can also select the action associated with the original object. Actual object data will be lost, such as modifiers.
As most addons are not working yet, we only have two export options in 2.80, collada and alembic. Both can export data that can then be imported in 2.79, when exporting you want to make sure modifiers are not applied so that you can re-create them.