There are many places online to build a DIY Hoffman apparatus. The ACS offers an electrolysis of water lesson that includes a hand made Hoffman apparatus(link is external) as part of a unit on energy that I used as a resource.
HS-PS1-3 Electrical Forces
During our recent chemistry summer camp, we used some electrochemistry activities. We had some logistical issues, but they were an overal success!
This is a series of experiments, PhET Interactive Simulation activities, and clicker questions to relate macroscopic and molecular representations of homogenenous solutions. Graphing skills are also used.