The term 'alternative to plastic' can be summarized by two crossword solutions: CASH (4 letters) and PAPER (5 letters). CASH is a form of payment that does not require the use of plastic, as it is a physical currency. PAPER is also an alternative to plastic, as it is a renewable resource that can be used for packaging and other items. Both CASH and PAPER are viable alternatives to plastic, which is becoming increasingly problematic due to its non-biodegradable nature.