Horsechestnut leaves can be ground with alcohol to extract esculin, a fluorescent dye. Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2. Mood lipsticks undergo many interesting pH-related color changes. See a related question for more. See the Canadian Society for Chemistry's Discover Canadian Chemistry site for more.
Onion is an olfactory indicator.
The onion odor isn't detectable in strongly basic solutions. Red onion can act as a visual indicator at the same time. It changes from pale red in acid solution to green in basic solution. Pansy petals Petunia petals contain petunin, an anthocyanin that changes from reddish purple in acid to violet in basic solution. Poppy flower petals Purple peonies contain peonin, which changes from reddish purple or magenta in acid solution to deep purple in basic solution.