They are completely two different things!
Apache Camel is a lightweight integration framework which implements EIPs (Enterprise Integration Patterns). I assume you know where to get more info about Camel. You use Camel to integrate your back-end enterprise applications. The outcome of Camel is an ESB solution to orchestrate flows between your applications. Don't be confused about the flowcharts in Camel. They are for designing messaging routing, transformation, etc, not BPMN flowcharts.
On the other hand, Activiti is all about Business Process Management. I assume you can differentiate Business Process Management versus Enterprise Integration Patterns. With Activiti, you can design and implement processes that improve your organization's business processes effectiveness and efficiency. Again, it's about managing your processes.
We are designing a case management system where our workflows are very generic (May 3 linear hops)
Can you elaborate more on your use case? What is your case management system? What does it do? What are the workflows?