The CMoS advises italics for video games, so:
"I stayed home and played Halo."
If italics are not available, you can use slashes:
"I stayed home and played /Halo/."
EDIT: Requested references:
In media where italicization is not possible, alternatives are used as substitutes:
In typewritten or handwritten text, underlining is typically used.
In plain-text computer files, including e-mail communication, italicized words are often indicated by surrounding them with slashes or other matched delimiters. For example:
- I was /really/ annoyed.
- They >completely< forgot me!
- I had _nothing_ to do with it. (Commonly interpreted as underlining, which is an alternative to italics.)
- It was *absolutely* horrible. (Commonly interpreted as bold. This and the previous example signify italic in Markdown.)
To use italics in plain text email messages:
Put a slash character before and after the /italicized passage/.
For bold, surround with asterisks:
She sells *seashells* by the seashore.
For italic, surround with forward slashes:
The rain in Spain falls /mainly/ on the plain.
For underline, surround with underscores:
I'm a mother pheasant plucker; I pluck mother pheasants. I'm _the most pleasant_ mother pheasant plucker who ever plucked a mother pheasant. Signed, Ophelia Bhutt.
Note: without the plain text formatting, some of these examples were automatically formatted by the SE engine.