Since I'm one of those who would qualify for your reunion, I feel fairly confident in giving you a few pointers: Most, if not all, of us were not into disco - Grand Funk Railroad or Led Zeppelin or Pink Floyd would be more representative, in my opinion. Flowers and smiley faces were HUGE, ditto anything "organic". Anything referring to peace - peace signs, the word "peace", "Make love, not war", the hand sign for peace - ANYTHING is good (the Viet Nam war was a big deal for us - still kind of is). If you need some visual cues, the Austin Powers movies (while over the top) kind of nailed it.
I did a search for '70s artwork and this came up: Image Search Results for 70's artwork
dafont.com has a few good fonts that you could use to print your own sentiments.
I like Keep on Truckin' FW Font | dafont.com
and Bell Bottom Laser Font | dafont.com
and LMS Hippy Chick Font | dafont.com
A lot of people were experimenting with pot and LSD, so psychedelic images were huge, neon colors made their debut, etc. Tie dye was big, mushrooms were everywhere.
Inkadinkado has a set of peace signs: Peace Signs - Inkadinkado
I have the CTMH Groovy stamp set, but also found one on ebay: https://chrome.ebay.com/tablet/item?...&sk=SCH_472065
Hopefully some of this overly-long answer will be helpful...