Doing something from used parts has great educational value. In the process you can learn how things are done, improve your soldering/desoldering skills, testing components techniques, encounter ICs and components you usually do have not even known to exist. Identifying components is an art too.
If you lack a project, take something out of junk pile and take the most intriguing part (just do not take into account large surface-mounted ICs: they are hard to reuse) and try to think how it can be used.
In my experience, however, it's hard to do a project without new parts altogether. Also, it may be harder to assemble reused parts on breadboard,
because they have much shorter "legs".
Unless you reuse the whole blocks (like power supply), design for the new project can be very hard if you restrict yourself to reused components only. Try to envision and implement a project with 2-3 interesting reused parts (for me those are usually LEDs, sensors, some rare precision resistors, potentiometers, battery holders, switches, boxes, standard connectors, ferrites, cables).