It depends in part on just how accurate you want. I believe @cdikland had a guitar storage project which required high humidity accuracy, he may have some suggestions.
I know some members use the Kumo wireless tags. These are small, with high accuracy, but are expensive. You have to get both a $34 tag and their $39 ethernet bridge. ( One bridge can handle up to 40 of their sensors) They don’t integrate directly with smartthings, but they do have an IFTTT channel, or there is an integration that a community member created that is quite popular.