A function generator, the 2-channel scope and a set of circuits to test. Connect the probes to circuit nodes, drive the input, and measure what really comes out: frequency response, phase, resonance, gain, bandwidth, slew rate, switching ripple, load transients, and cable faults with time-domain reflectometry.
The generator has a 50 Ω output. Its amplitude display assumes the load you tell it about (High-Z or 50 Ω): if the real load is different, the real amplitude is different. For frequency response, measure input and output at the same time (CH1 on IN, CH2 on OUT) and use small signals so nothing slews or clips.