Can you do anything active if you have pericarditis?

Anti-inflammatories. Anti-inflammatory agents such as nsaids, steroids or Colchicine can help in the active state of pericarditis.
Pericarditis. Technically speaking, yes. In other words, no physical activity will make it worse. On a practical level, however, the discomfort or pain associated with pericarditis may limit your ability to exert yourself.