Hot foil
A metallic film stamped onto the surface with heat. Gold and copper are the usual choices, and it is genuinely reflective in a way printed metallic ink is not.
On metal you have an alternative worth knowing about: leave the area unprinted and the tinplate itself shows through, which is already a mirror. That costs nothing and it is often better than foil on a tin.
Spot UV
A clear gloss varnish applied to selected areas only. On a matte tin it gives you a shape that appears and disappears as the pack turns in the light.
It works hardest as a subtle move. A spot UV logo on a matte black tin does more than a spot UV pattern over the whole face, because the contrast is the effect.
Matte varnish
A flat sealing coat over the print. It kills reflection, which makes colours read deeper and stops the pack throwing highlights everywhere.
It also takes fingerprints more readily than gloss. Worth knowing for a dark pack that gets handled a lot, and worth ignoring for a pale one.
How many to use
One, usually. Each of these is a contrast effect, and contrast needs somewhere quiet to work against. Foil plus spot UV plus emboss on the same face gives you three things competing and none of them landing.
The packs that look most expensive tend to be a matte ground with exactly one thing happening on it.
Hold one before
you specify it.
Tell us what you're packing and we'll send the formats that suit it.