Whats the safest way to return to activity after my nondisplaced hairline fracture of my tibial plateau heals? Is a stationary bike safe?

Return Slowly. Once a fracture is healed you can resume full range of motion activities of the knee. Isometric exercises (like quad sets) can be the start. Partial weight bearing activities are first like: running in a pool or stationary cycling. If after 4-6 weeks these activities cause no discomfort you can resume full weight bearing activities like running . But start slow on a tread mill s best.
Low impact exercise. After being cleared by your treating doctor, start with low impact activities. Cycling and swimming are two good options.