How to fix/cover interior hollow core doors without replacing, ideas?

The cheapest method is simply to pain them. You can use many forms of stencils or geometric patterns with paint if you do not go for the plain single hue. Get creative and use a highlighter to emphasize the molding or shapes built into the door. Secondly, there are also large poster sized views of nature or animals such as a wolf or a bear available to put on walls that can be adapted to a door. Use a relaxing photo sticker of the ocean, a mountain or a stream. If there are any damaged areas on the door, sticker posters can also cover these. Thirdly, depending on the look and how much time you want to spend, there are many wall decorations that can be applied to a door such as wood or rock look. These are very thin and light weight and many come with a sticky back to simply apply to a clean surface.

If they're plain slab doors, you can add molding to them to dress them up and then paint the whole thing. There are tons of ideas on Pinterest for dressing up interior doors.

Make 3 rectangles with picture molding on the door on both sides. Glue and nail with finish nails. Fill in cracks and nail holes with caulk. Then paint the whole door, front and back.

If they are damaged, cut and glue on a piece of luan and trim it with molding.