This lab is a hit every year with my students! I use 4 plants per each class period, and they monitor them.
You could easily adapt so that students work in small groups or individually.
The file is fully editable.
Students essentially water 4 plants each day with "mystery liquid." (4 water samples that you have prepared with various pH levels) and observe the effects on the plants, measure the plants heights, and create a graph from it. Usually within 7-10 days there is a pretty obvious difference (varies depending on how acidic you make the water). Students water the plants as a warm up, and we move on to an acid and base lesson or lab for the day.
After the lab is complete, students interpret their data and make predictions as to what samples contained acid, how it affected the plants, etc. I usually follow this with a project where students make an acid rain brochure to educate people about the causes and effects of acid rain.