Look at common DIY escape room ideas for inspiration. Good 70s puzzles include using a Fortune Teller/Cootie Catcher to reveal numbers or using "secret agent" ciphers inspired by the Cuban Crisis or MI6 themes.
Design a paper vinyl record where the grooves or track names hide a message. For example, the first letter of every third song title spells out a clue. Escape.from.the.70s.rar
Create a printable rotary phone dial . Players must "dial" a specific word (like "CALL ME") found from other clues to reveal a code. Look at common DIY escape room ideas for inspiration