My old tube radio has a gramophone input that acts like an auxiliary in. There I injected the audio output of a cheap 9$ bluetooth A2DP audio receiver from china (dealextreme) and connected a standard 5V power supply unit which is switched on together with the radio. This way I got my personal handmade bluetooth speaker. I really like the warm sound of these old radios and feeding it with a modern cell phone feels just super cool.
Attention: Be careful if you also want to upgrade an old radio! Most of them run directly on 230V. Be sure you know what you do!
This project also shows some sort of implicit critics against the just-so-common planned obsolescence. Do you think the stereo you bought just a couple of months ago will still work in 50 years? I guess not.