Probably something like cocktail attire? Business casual? We had a park reception and tried to tell all our guests to wear comfortable shoes LOL since they would be doing a LOT of walking! some people didn't get the memo though! LOL We said comfortable clothes, but people in our family wouldn't dream of being super casual to a wedding, so we weren't worried that they would come in jeans and tennies...for example, my aunt's wedding they said "resort attire" (casual), and everyone dressed like they typically do for a wedding, suits and ties, dresses...
p.s. we didn't state it anywhere on our invites...