Watches

The traditional rules would be that with formal dress you should be wearing a leather strapped dress watch (or not wearing a watch at all).

But that is following the traditional rules, and assuming you are dressing in a fully formal fashion. In reality there are many different grades of suit and levels of formality for different situations - taking a slightly more casual approach then a nice sports watch on a bracelet (think datejust/oyster perpetual or aqua Terra) would be a reasonable choice.

But it is also worth noting that the world has moved on since those rules were formulated - whereas once the expectation was that every detail of your dress would be immaculate, often you well find a slightly more casual approach nowadays, and the true formal wear is really reserved for special occasions. I don't doubt these events do exist, but I can only say that I have never personally been invited to one...

The clue is generally to look at how those around you are presented. If every one of them is perfectly dressed and there are traditional dress watches all round, then you may be the odd man out. But not realistically, if you can see a scattering of smart watches and fitness trackers hidden under cuffs, then a nice sports watch on a bracelet is going to be absolutely fine (if not actually smarter than most).