What do you get a mountain biker who has everything?

Things that wear out (brake pads, sealant, gloves), tools they would not buy themselves (a pressure gauge, a bike stand, a torque wrench), experiences like a bike park day or skills clinic, or a membership in their local trail association.

Is a trail association membership a good gift?

Yes. Most trail networks are built and maintained by volunteers and funded by dues and donations, so a membership supports the trails the rider uses every week.

What is a good small gift for a mountain biker?

Tubeless sealant and valve cores, a set of brake pads for their exact brakes, gloves or a digital tire pressure gauge.

Should I buy bike parts as a gift?

Only consumables you can match exactly, such as brake pads or a chain for their drivetrain. For bigger parts, give a shop voucher instead.

Gifts for mountain bikers who have everything

The short answer

A mountain biker who "has everything" has everything that lasts. They do not have new brake pads, a full tubeless kit, a free Saturday at a bike park, or a trail association membership. The best gifts for them are things that wear out, things they would not spend on themselves, and experiences.

Things that always wear out

Unexciting, guaranteed to be used:

Brake pads for their exact brakes. Check the brand and model on the caliper. Riders always need a spare set.
Tubeless sealant and valve cores. Sealant dries out every few months.
A new chain. Only if you know the drivetrain (number of speeds and brand).
Grips. Only if you know which ones they like. Ask a riding friend.
Gloves. The pair they ride in now has holes. It always does.

Things they would not buy themselves

A really good floor pump or digital pressure gauge. Most riders guess their tire pressure. A gauge stops the guessing.
A bike stand for the garage. Makes every repair less miserable.
A proper torque wrench. Carbon parts and modern bikes need the right torque, and most people use "tight enough".
A shirt that is about riding but does not look like kit. Most riders own a drawer of jerseys and nothing to wear to the pub afterwards. For e-MTB riders in particular, the Watts tee is a light polyester tee they can ride in, with a chart on the back that ends in "watts are overrated". It is $55.95, ships free, and is printed to order, so allow up to two weeks in the US.

Experiences

For someone with a full garage, this is usually the best category:

A bike park day with lift or shuttle tickets. Check that their bike is allowed (some parks restrict e-bikes).
A skills clinic. Cornering, jumping or steep technical riding. Even experienced riders get faster after one session.
A guided ride somewhere new. A local guide knows the trails that are not on the app.
A weekend trip. Pick a destination with good trails, book the lodging, let them plan the rides.

Gifts for the trails, not the rider

Riders who have everything often care most about the places they ride:

A membership in their local trail association. Most trail networks are built and maintained by volunteers and paid for by dues and donations. A membership in their name is a gift that pays for dirt.
A donation to a trail-building project. Many clubs fund specific new trails and will often credit donors.
A volunteer dig day together. Free, and you get to see what goes into every trail they ride.

Quick picks by budget

Under $50: a set of brake pads for their exact brakes, tubeless sealant and valve cores, a digital pressure gauge, or a year of trail association membership (dues vary by club, but many are in this range).
$50 to $150: a skills clinic, a quality floor pump, a torque wrench, or a shirt they can ride in and wear afterwards.
Over $150: a bike stand, a bike park day with lift tickets, or a guided ride somewhere new.

If you are buying with other people, pool the money into one experience rather than three small things. A day they remember beats a drawer of accessories.

What to skip

Big components you are guessing at: wheels, forks, shocks, dropper posts.
Helmets and shoes without a fitting.
Novelty gadgets that will end up in a drawer.

If you can only ask one question

Ask: "What is the most worn-out thing on your bike right now?" Then buy that, or the shop voucher to replace it. If they say "nothing", book a day out on the trails.

For e-bike riders specifically, there is a separate list: gifts for e-bike riders.