Well, you can't get simpler than a harmonic doubler.
Starting from a square wave, you'll need to filter it to something sinusoidal, then chop it with some offset so as to produce a different duty cycle (ideally 25%). Typically a class C amplifier will do the job, giving a lopsided sinus thing -- rich in even harmonics. A tuned load selects the 2nd harmonic, then is followed up with another amp (if possible, a limiter, to remove amplitude modulation -- the intermediate signal will be double-humped as it's pumped by the first amp's pulses), then whatever output conditioning the receiver needs (buffering, filtering, level shifting, etc.).
Now, that's less than a half dozen transistors all in, but it's the passives around it that are key. How much complexity are you looking for in terms of tuning versus It Just Works(TM)?
If you need something fancier, you'll need a PLL. You may prefer a PLL anyway, just because you can get a multiplier or synthesizer chip for a few bucks, that's many hours less time building and adjusting than the classical equivalent.
Tim