Arriving at a party with your own drink in hand solves about eighty percent of the social friction of being sober at a drinking event. The host does not have to think about what to offer you. You do not have to negotiate with whatever the open bar happens to have. You walk in already settled.
What follows is practical and specific.
Why you bring something good
Bringing a can of seltzer is fine. Bringing a bottle of something that looks intentional is better. The visual signal "I brought my drink, I am set" is what the room reads. The drink does not have to be elaborate. It just has to look like a choice, not a substitute.
The hosts often appreciate the gesture. Most hosts of drinking events end up with a few non-drinking guests and have to scramble to accommodate them. You handling your own drink is a small act of generosity to the host. Many will adopt your bottle into the kitchen offerings for other non-drinkers, which spreads the option.
The categories
Sparkling waters with botanicals. Topo Chico with citrus slices is the workhorse. Liquid Death's flavored sparkling water hits a slightly more aggressive aesthetic if that suits your room. Spindrift's grapefruit is excellent over ice. None of these read as kid drinks.
Non-alcoholic spirits. Seedlip's Garden 108 mixed with tonic and a cucumber slice is indistinguishable from a gin and tonic at three feet. Lyre's Italian Spritz mixed with sparkling water and ice over a slice of orange reads as a spritz. The price is higher than soda but lower than wine, and a single bottle lasts several events.
Non-alcoholic beer. Athletic Brewing's lineup is unrecognizable from real beer in most settings. Their hazy IPA in particular passes scrutiny. Bring a six-pack and put it in the fridge with everything else.
Non-alcoholic wine. Less established than the NA spirits category and more variable in quality. French Bloom's sparkling is the most reliable; their blanc tastes like an actual brut and pours like one. Avoid the cheaper supermarket NA wines if you can; the gap between best and worst is large.
A kombucha or shrub-based drink. Acid-forward, slightly bitter, more interesting on the palate than soda. Reads as a deliberate choice rather than a default.
What to skip
Anything sweet. Sweet non-alcoholic drinks read as juice and signal "child" or "designated driver." Both readings are fine if that is what you are going for; otherwise skip.
Anything pink with a straw. Mocktails as served at restaurants often skew toward visual presentation that reads younger or more performative than you want.
Anything that requires explanation. If you have to tell the host what it is and how to serve it, you have undermined the bring-your-own move. Stick to formats people recognize: bottle, can, six-pack, growler.
The arrival move
Walk in. Greet the host. Hand them the bottle or can of sparkling water you have already opened, or offer to put your six-pack in their fridge. Say "I brought this, it's what I'm drinking tonight, want one?" The "want one" is the move. It transfers the framing from "I have a special diet" to "I brought something interesting that I'm sharing."
About a third of the room will take you up on it. A few will switch over for the night. You are now the person who introduced the room to something they had not had before, not the person who needed an accommodation.
The drink-in-hand effect
Throughout the rest of the event, keep something in your hand at all times. An empty glass is a question. A full glass is an answer. The question, "what are you drinking," is much easier to answer when you have something specific in your hand than when you do not.
If you ever find yourself sober at a party with empty hands, refill immediately. You will be asked less, you will feel less observed, and you will be more inside the room than outside of it.
Cost note
A reasonable starter kit is two bottles of NA spirits, a case of good seltzer, and a six-pack of NA beer. Total cost is around forty to seventy dollars at retail. The kit lasts months of events. The first time you arrive at a party set up like this, you will not go back to figuring it out on the fly.