When I'm feeling like not drinking, I usually go for ginger ale or ginger beer, but they're quite sugary. Cutting them with water or a lot of ice can help. Juices universally are pretty sugary too so cutting with them is delicious but not really healthier.

My other go to is soda water with bitters or a small splash of bitter liquor. Bitters are high alcohol but you only get a few drops at a time which is comparable to "0 proof" drinks which can have some trace amounts of alcohol still. Many bars have a variety of bitters and aperitifs so its a bit like having an herbal tonic soda jerk. A squirt of lime or other citrus, or a few syrupy cherries, will also add to such a mocktail.

To be fair, beer and liquor are poisons, and they have tons of calories from unfermented sugars and the alcohol breaking down into more carbs in your body (hence beer guts). If your goal was to lose weight, stick to water. But if it's just to cut back on booze, juice and sugar soda aren't so much worse for you.