One classic experiment you can try is very simple but the parts can be tricky to find:
You need a Gunn diode oscillator and mixer as used in automatic door opening equipment. If you can find one second hand they cost pennies, I'm in the UK and they can be found at electronics fairs and junk sales for around £3 ($5 US), often with the seller having no idea what they are. They look like a metal block with two insulated pins and a rectangular waveguide slot on one end. When about 9V is put on the pin furthest away from the slot, they start to produce microwaves at around 10.7GHZ. The second pin is the mixer output, basically a connection to a diode mounted a few mm inside the slot.
The trick is to measure the current from the mxer diode (the metal case is the ground connection), it will be a few 10s's of uA. When an object is moved in close proximity tio the slot, the current will vary, this is what the door opener uses to know somebody is aproching. Now place a flat reflector in front of the waveguide and move it back and forth. Part of the output power is internally coupled to the mixer, part comes from reflections back into the slot from outside. As the reflector is moved, the phase of the reflected waves reaching the mixer will change but the fixed path inside the unit will remain constant so the waves will interfere constructively or destructively depending on the reflector distance. The nice thing about this experiment is it is compact and by measuring the distance between peak and trough curents, the wavelength and therefore frequency can be calculated.
Brian.