I like the Australian approach. We almost always form a government quickly with the Senate acting as a reasonable check on legislative adventurism. The poms have it better with the House of Lords, but I don't think a similar institution can be engineered ab initio, it's an evolved body. The Canadians tried, I understand they don't regard it highly.
I'd fiddle with the Senate formula to allow more minority parties into the mix (for the past few decades it's settled into a stable tripartite pattern, 2 majors and a minor).
Speaking as a Canadian, the Senate is little more than a retirement program for friends of the party (which party is of little consequence). The party that currently holds power spent years talking about Senate reform, only to revert to the status quo once they got into power. Thus, the Senate rubber-stamps legislation, but no one seriously discusses what goes on in that body, because I think that it is generally understood that it doesn't make a difference.
I'd fiddle with the Senate formula to allow more minority parties into the mix (for the past few decades it's settled into a stable tripartite pattern, 2 majors and a minor).