Decorate with paper bunting, fabric banners you reuse, and real or borrowed crockery. Never release balloons, latex or foil.
A party set of plastic plates, cups, cutlery and balloons is easily 200 g of plastic; a few parties a year is around 300 g. Plastic plates and cutlery are already banned for sale in the EU, UK and most Australian states.
The catch: latex balloons are biodegradable in months but still kill wildlife before they degrade, so latex is not a free pass; foil balloons are plastic film and should be avoided.