While you could continue your training by taking the Senior System Architect and diving deeper into the core Pega Platform you could supplement your training by taking a look at the CRM Applications that Pega provides to create world-class applications. I would start with the following courses.
Going through these training offering will give you the Customer Service Ready Badge. Are you working in a particular industry? Finance, healthcare, insurance, telecom, etc? If so, there are different foundations courses available for the various different verticals.
I'd also recommend taking a look at the Pega Decision Consultant course. This will equip you with the skills to build Next-Best-Action strategies that you might combine with the Customer Service Application to create some rather robust applications.
If that is not enough, you could also take a look into the Pega Marketing Consultant course.
I believe these courses will allow you to see how Pega can be extended and used in the real world to solve real problems. The sky is the limit, there are over 230 courses available on Pega Academy. I hope I have provided you with some next steps.
Great. As far as getting more experience with activities you can look at Java and Activities and Data Pages on the PDN. You'll notice there are number of articles under those topics which will allow you to test functionality on your exercise system.