Liquor provides a cocktail with more than ethanol (though ethanol is the main attraction). It provides body. It provides a burn. A good no-proof drink also has a backbone of some kind. Products like Seedlip seek to replicate the botanical support provided by gin and other spirits, but I wasn’t super impressed with the stuff when I tried it (though I have developed a way to make your own fake spirit, which I will get to in a bit).
Unsweetened fruit juices and strongly-brewed teas can work as bases for your mocktails, but sweet and vinegary shrubs are my favorite alcohol-free beverage foundation. The acid gives the drink a bite, and they look pretty when mixed with soda water. (They’re also very good with black tea.)
If you want something with a little more botanical quality, you can make “fake gin” with flat tonic water, dried juniper berries, a splash of rosewater, and a few other plant parts. Let everything infuse for a few days—or speed it up with your immersion circulator—and mix two ounces of your fake gin with one ounce of fresh lemon juice and 3/4 of an ounce of honey syrup (or use any of these Camper English Seedlip recipes, subbing in your own creation in place of Seedlip).