What to do with an old PDA HP ipaq rx5700?

to give me)

What's the point? Such a device place in the Museum. Wasting time on porting it anything makes sense only for their own self-education.

Use similar as a video player, a notebook and calculator. Given how popular Android s etc., not very modern, but cheap.

If you have a cradle to use as a photo frame :)
Well, if you still want to put Linux, I advise you to dig in the direction of the Familiar Linux
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Familiar Linux is a replacement for Windows CE on iPAQ machines and other personal digital assistants (PDAs). It can use OPIE or GPE Palmtop Environment palmtop environments. Familiar Linux is a complete operating system with many applications.

It has wi-fi?
Two options:
1. to install skype or sip tonelico and use as a phone.
2. if there is a webcam above the door in the hallway, then attach the PDA to the wall (to bring food) and open a browser with the image from the camera.
Both versions have long plagued me, but not going to try.
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If there is no wifi, then give it to someone, and wishing already have! :)

If it is liquid crystal, the display elements it is possible to make a diffraction grating!