If you got compose key (linux, mac, windows with third party software), then those are trivial:

ë ñ ũ ü, and even åâăāãȧaąàáæª₂2²

Goes like Compose e ", Compose n ~, etc

But a thing to note that resulting letters are generic and not region-specific,

like that ë (U+00EB LATIN SMALL LETTER A WITH DIAERESIS)

is not the same as ё (U+0451 CYRILLIC SMALL LETTER IO)

Which might trigger spellcheckers or not even be displayed in certain fonts

There's also apparently some weird combos like Compose+:) for ☺ and Compose+CCCP for ☭, but no easily available keys for greek letters unless you tweak configs...