Are you intent on making a large downpayment versus going FHA with 3% down? That's how I did my home. Fixed interest rate was very reasonable for the market at the time even after PMI. And an FHA mortgage is "assumable." So you can lock in a low fixed rate, and sell the house and mortgage as a package if you move later. Buyer might be willing to pay more if you can get them a 4% rate in a 6% future rate environment?
There aren't really any "good" no risk, high yield instruments right now - thank the Fed. If you insist, I'd look into 3 month CDs. The rate will be low but maybe a little better than the advertised rate if you are securing 30K in a single instrument.
If you are going to keep it a little longer, US Savings Bonds might work. There is usually a 3 month interest penalty if you sell sooner than 6 months holding period. The investment is guaranteed though and exchangeable for cash at any FDIC bank. There may be limits on how much you can put into each series per year...
Personally, I'd be content to just hold cash in a regular savings or MM checking account at today's rates, IF I was going to use the money as DP soon. Half a percent more over 3-6 months is going to be trivial.