In the category of Better Late than Never, National Volunteer Week was, and is for another day and a half, this week: April 15-21. At least some places are free through Sunday April 22 as well.
With proof that you're a volunteer (e.g., volunteer ID badge) some museums offer free admission, and there may be other benefits as well. Check with your local entities, and please post to the wiki if you find anything useful, especially since this year's list may well apply in large part to future years.
Future National Volunteer Week celebrations: April 7-13, 2019, April 19-25, 2020, and April 18-24, 2021 - which gives you plenty of time to become a volunteer if you aren't already.