Mountain bike -- exciting after-market parts, modifications or paint jobs
Let's create something of a (price) ordered list:

It all depends on what kind of look you are hoping to achieve.  I would suggest if the bike is a neutral color like yours, you try to find a detail color from, say, a decal, and replicate that in various places over the bike by hunting down the parts that match.

This process is an endless rabbit hole, after which you realize you would have been better off building your own.

You can even go the best function route, which may end up looking inconsistent to the uninitiated, but you'll know it looks good :)

Almost Free


Valve caps
Clean and Polish!


Cheapest


Grips, check
Brake discs
Brake pads (caliper brakes)
Colored bolts (but be careful - because they are normally aluminium and significantly weaker)
Handlebar end caps
Different color chain
Handlebars


Middling


Tyres
Saddle with Shiny bits or Brooks, according to preference
Cranks and rings, shiny, anodised or matt, machined or forged
Frame repaint (strip, sand blast, powdercoat or automotive paint, sealer/clearcoat)


Crazy


All the parts have to be made of Titanium and/or Carbon Fiber


I'm sure there's plenty more that I've just forgotten.