Can anyone tell me where to donate a lot of Hero Factory parts and pieces?
If I were you, I'd try one of the following:

If you wish to get some money for them, post them as an auction on eBay without a minimal (reserve) price. This way somebody will surely bid at least some amount, and the lot will be off your hands and on it's way to someone who actually wants it at least a bit.

Posting on local marketplace boards (like Craigslist, Facebook Marketplace, etc.) could attract some interested recipients as well, either for money or for free.

If you don't want to get any compensation for it, you could try local kindergartens - I suppose they would welcome additional toys for the kids there.

If all else fails, you could just pack them up in a bag, attach a note abut what it is and what are your intentions, then leave the bag in a playground for someone to find. This is the most risky option, as parents in your area might be rightfully hesitant to let their kids pick up anything remotely suspicious, but it surely beats throwing the lot in the trash.