Enable the Compose key and you'll get even more easy symbols, and they're reasonably guessable.

  Compose ` e produces è
          " a produces ä
          v s produces š
          v S produces Š
          a e produces æ
          C = produces €
          l - produces £
          - > produces → 
        ( 1 ) produces ①
          ^ 1 produces ¹
          _ 1 produces ₁
          1 8 produces ⅛
        - - - produces —
        - - . produces –
          . . produces …
          . - produces ·
          | - produces †
          | = produces ‡
          " < produces “
          x x produces ×
          m u produces µ
          > = produces ≥

See /usr/share/X11/locale/en_US.UTF-8/Compose for the list and https://en.wikipedia.org/wiki/Compose_key

I have also configured Shift+Compose to send the code 'dead_greek' using ~/.Xmodmap:

  keycode 135 = Multi_key dead_greek Multi_key Multi_key

Then I can type α, β, γ, Δ, Ε, Ζ easily, although I hardly ever need this nowadays.