US Treasury Bonds are currently yielding 5.5% for short terms - - unfortunately no investments can ever guarantee a future...
It is nice that you have concerns for your wife - - I do too and have found that the best thing for her is to put money in her name now. And through the years, she has done great, gradually buying farmland - - and as the land has appreciated tremendously, the family has profited through farming...
The recommendation of VOO etc is also a good one but being older, you might want to consider that you might not have the time for a longer term investment. I will soon be buying into that fund but at a much smaller % of my funds than I did when I was younger.