It's hard to give meaningful suggestions without knowing your level of equipment or experience, as well as the parts you have on hand, but here's mine anyway.
Make a Wein bridge oscillator. Use a flashlight bulb in the feedback loop, the way Hewlett did in the original HP200A. See how pure of a sine wave generator you can make, and you might consider making its frequency adjustable over a very wide range.
After that's done, if you want more, modulate the gain and make an AM radio transmitter out of it.