If you can get your hands on some true goldenrod paper, you'll have yourself an indicator paper.
Goldenrod paper turns bright red in the presence of a base.
Dissolve some baking soda in water or use ammonia to paint and vinegar to erase.
I know a high school science teacher who, on Halloween, would pretend to cut himself, "wash" his hand (with ammonia) and fling his hand at a sheet of this paper hanging on the board. Totally freaked his students out! He lists several other demonstrations using this paper on his website.
Now, not all goldenrod paper is true goldenrod (i.e. it doesn't contain the real dye/extract), so you'll want to test yours out before passing it along to your students. Astrobrights Galaxy Gold paper
does work, so look for it at office supply stores if you're having a hard time finding paper that works.