Get a projector with at least 1080p actual output resolution.
Most entry level ones have terrible lenses which means you'll need to experiment a bit with how high you'll mount them cause they need to be dead on to display properly, otherwise half the screen will be blurry. I highly recommend a cheap aluminum camera tripod with a center collumn for height adjustment.
For screen I bought a white roller blind for like 8 bucks from a home supplies store and mounted it by passing a string through the tube and tying the ends to whatever worked best. In terms of picture quality it's exactly the same, but if you can afford an actual projection screen and haul it around then do that.
For sound I would recommend a battery powered Bluetooth speaker because they're cheap and easy to carry, but you may experience a slight sound delay which can be solved with a 3.5mm aux cable in the aux port.
My final suggestion is to get a used Nintendo Wii with sports and a couple of controllers, endless fun for the right crowd.