Cover the underside of the top bunk mattress with a fitted sheet. Then, flip the mattress over and make it as you normally would. The bottom bunk will have a view of a sheet instead of an unsightly mattress bottom. Use colors or images preferred by the bottom bunkmate.
Lay panels of fabric, flat sheets or curtains across the top bunk before you put on the mattress. The fabric can dangle over the sides of the lower bunk, creating a canopy. Tie the fabric with simple strips of the same fabric or let the fabric cover the lower bunk for privacy or to create a fort. Dangling fabric may not be suitable for young children.
Paint a piece of plywood cut to fit the dimensions of the top bunk. Lay the plywood on the top bunk, then the mattress. The painted plywood could be a solid color or feature, painted designs or stickers such as glow-in-the-dark stars.