Some tiny little 512-byte bootsector programs. Should be able to start from any bootsector, whether it is floppy or hdd or USB.
A big fullscreen digital clock. Can be switched between 24 and 12 hour format, and has adjustable foreground and background colors.
nasm -f bin clock.asm
Then try it out for example on Qemu:
qemu-system-i386 -fda clock -boot a