\documentclass{article}

\begin{document}
Hello w\`{o}rld!

Hello w\'{o}rld!

Hello w\^orld!

Hello w\"orld!

Hello w\H{o}rld!

Hello w\~{o}rld!

Hello w\c{o}rld!

%Hello w\k{o}rld!

Hello wor{\l}d!

Hello w\={o}rld!

Hello w\b{o}rld!

Hello w\.{o}rld!

Hello w\d{o}rld!

Hello w\r{o}rld!

Hello w\u{o}rld!

Hello w\v{o}rld!

Hello w\t{or}ld!

Hello\_world!

and some real superscript, e.g. a degree sign:

Water: H$_2$O
\end{document}

