The bulk of our mid-term savings ("x" months worth of essential living expenses) is in an account with Capital One through Costco's website. The interest rate is higher than our ING account (short-term savings) and the CapOne account earns a quarterly bonus and we received cash for opening the account and funding it within a period of time (which we left in the account).
The rest is invested in a variety of ways (stocks, mutual funds, ETFs, etc.).
DH & I had 401k accounts with certain jobs and rolled them over and they are still invested and growing. Neither of us has access to a 401k now, though, so we can only invest $5K per year each into our Roth IRAs for retirement.
While our mid-term savings is intended to support us in the event of job loss or other major need during our earning years, it will also help support us in our retirement years. We need this money to grow, so I invest a percentage of it. I have a degree in finance and worked in securities and banking for years. My risk tolerance is higher than average. YMMV