If I tell it to walk ahead of me, stomp on the floor (for pressure plates and covered pits), and swing a stick up and down (for trip wires), does it weigh enough to trigger most traps?
Put another way, what are the limits of the traps it can trigger?
Unseen Servant is shapeless and made of force that can interact with objects:
This spell creates an invisible, mindless, shapeless, Medium force
...
Once on each of your turns as a bonus action, you can mentally command the servant to move up to 15 feet and interact with an object. The servant can perform simple tasks that a human servant could do, such as fetching things, cleaning, mending, folding clothes, lighting fires, serving food, and pouring wine.
What are the downsides? The Unseen Servant can interact with 1 object per turn, if you want it to have a stick for tripwires and a rock for pressure plates, then it would have to drop the rock, pick up the stick, sweep, drop the stick, then pick up the rock again. That's a slow process for each 5ft square. Anyone nearby would definitely be aware of the rhythmic tapping and thumping. You could have the servant press down on the floor and sweep the stick carefully to avoid tapping, but even so it's certainly going to make slow progress.
Wording of the spell:
The servant can perform simple tasks that a human servant could do, such as fetching things, cleaning, Mending, folding clothes, lighting fires, serving food, and pouring wine.
Given that most traps are rigged to set off with reasonably light interaction I think it is reasonable to assume that if the unseen servant can "pour wine" it can trigger trip wires and the like with its movements.
You mention "stomp on the floor" which is a more interesting problem as can a shapeless thing stomp? But it could be instructed to move along a corridor pushing down on tiles and flagstones. Even with a strength of 2 that should be sufficient to trigger something that was set to trigger when someone of normal size walked over it.
It's an "invisible, mindless, shapeless force", not a corporeal being. It interacts with its surroundings by grabbing or pushing on objects, which it does with a Strength of 2. This gives us some idea what it can do: assuming it works like a Medium creature*, it can carry 30 pounds and push or drag 60 pounds.
Since it's insubstantial, it couldn't "stomp" on a pressure plate to set it off, but it could push against the floor with 60 pounds of force. This should be enough to trigger any trap that would catch, say, a gnome.
A more efficient use of its strength would be to push a handcart full of rocks. The weight limit for a wheeled vehicle is five times your carrying capacity, so it can push 150 pounds, which should trigger any pressure trap that a single humanoid could set off. This will also detect tripwires, since the cart's wheels are touching the ground. Of course this requires you to drag a cart around, but if it survives its use as a trap detector, you can also use it to haul treasure home.
*It's not a creature at all, of course, though the spell does compare its range of possible tasks to a "human servant". Absent any other indication, I'd rule that it uses the same size class as its caster, so that it's useful in the same way whether it's a pixie or a giant casting it.
