I've talked to a few people recently who've been enthusiastic about swapping traditional keyed house locks out for combination locks, and while I do see the appeal they're somewhat expensive and require batteries. But they're definitely a more secure way to implement access conditional on "something you know" than hiding a key.

Another option to consider, however, is a combination key lock box, sometimes called a "realtor box":

You set a combination on it, and then store a spare key inside. It is more hassle than a pure combination lock for access, but for a our house it works well. People generally carry their keys, but we still have a reasonably secure option if someone forgets theirs or a guest arrives while we're out.