Sometimes, the simplest solutions are the best:
- Step on plastics to crush them - Crude, but efficient, and good for stuff like takeaway food containers, PET bottles (with lids removed), plastic cups, and even balled-up cling wrap. It isn't great for crinkly plastic like chip packets, but they can usually be flattened out by hand, and don't take up that much space anyway.
- Cut up larger pieces - Use a pair of sturdy scissors or wire cutters to cut up larger, tougher plastics into smaller pieces. This is what I do for blister packs and old/broken containers. A good pair of industrial scissors work well for a lot of home-grade plastic, and would be a good investment if you find yourself cutting up large bits of plastic often.
Any active/powered tool or system is going to use some amount of resources. Because you specify that you are only doing this as a pre-step to passing the plastic into the municipal waste stream anyway, it's worth mentioning that some systems will probably increase the overall carbon footprint of the pieces of plastic that you feed into it. So please carefully consider if this is enough of a problem for you to consider some sort of automatic system to handle it.
With that said, there is a potential option:
Warning: This will depend heavily on how your recycling center handles plastic waste - some may not accept shredded plastic at all (being reliant on finding the type of plastic by the symbols that are placed on it). Definitely double check with your local waste handler to see if this is an ok approach for them:
The 3D Printer community deals with this kind of problem with their test/failed prints, and has taken to shredding plastics using either store-bought or homemade shredders. In addition to motor-driven ones, you may be able to find or build manual shredders as well.
There are a few designs out there, ones that are similar to a food shredder, and others more similar to paper shredders. Note that any given design may not handle all shapes, sizes or types of plastic.