I've got emergency cash invested in SWVXX, a Schwab money market fund. If cash is needed, I can sell and then the cash is available the next day.
Share price never varies from $1/share (e.g., not subject to revaluation of bonds as interest rates move around).
I believe the way they keep the share price flat is by only buying short term bonds (like 1-2 months to maturity) and then holding them to maturity.