When just beeps and tones are not enough, adding a Liquid Crystal Display to your project will let you print out messages and information onto a small light up display.
These LCDs are in almost all electronics, and can display basic text as well as very very small icons like the heart and smiley face in this picture:
If you look closely you can see the little rectangles where the characters are displayed. Each rectangle is a grid of pixels. Normally all the pixels are blue, but when you tell the LCD to change them, they swich to transparent, and the white backlight shines through.
You can mix an LCD with sensors to display instant feedback, and when powered with a battery lets you have a project that can go portable.
The LCD also may come with an extra 10KΩ resistor which is used to change the contrast/visibility.