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).
Bittersweet Italian sodas are another good option, and you should snatch up any you might find in your area. Sanbitter Red is basically non-alcoholic Campari. Mix it with fake gin for a fake Negroni.