Credit unions aren't all amazing. I actually asked this question on behalf of my elderly mother, whose finances I'm now managing, because I want to move some of her savings from her credit union that pays an absurd 0.1% to a high-yield savings account.
I like the idea of a money-market fund such as Vanguard - thanks to RWD for the suggestion. Their "Federal" MM fund invests mostly in US government securities so it's pretty safe even though it's not FDIC insured. And one can easily buy brokered CDs or T bills. I figure Vanguard would probably also make it pretty easy to move money into and out of the account. In fact, I'm thinking of moving a chunk of my own savings from high-yield savings accounts at various banks to a MM fund, as it does seem to provide exactly what I'm looking for - high yield and ease of investing in other low-risk, high-yield securities.