If you're interested in encrypting an email with moderate strength but not willing to go through the PGP rigamarole, you could use a Vigenere cipher. It uses a previously agreed-upon word (the key) to apply a rotating Caesar Shift. If I applied the word "protect" to this paragraph, this is the output:
Xw mhy'tx xehxvglivr br ggrimixkgv rb xqcba nwml ohsvftxg liisgkva qlh gsv pxczbri md xc mlthjxv mlg IVG fbkcfpicei, ahj tcnpf nhv o Omixcvfx Gkiwvf. Bx wltj o ivgoxfilpa tvisxh-wide khvf (mwv yxc) vh pgdec c kdkommpz Rrslet Lwztm. Mh B pgdemgw iys pstw "eicmiem" if hamu ipiozvciw, kvbw kl iys hyvijk:
You can use this page to decrypt that or encrypt your own. Note that the Vigenere cipher is not invulnerable to cracking, but it's definitely stronger than rot13, and is a ton simpler than private-public key systems.