...that tomorrows tech is just a collection of encapsulated tech from yesterday.
So learn a couple modern web/mobile frameworks.
Their structure and workflow will show you current development trends and give you a taste of each piece before you take a deep dive.
For extra credit (and an enhancement to your adaptability), pick both a popular web stack and an obscure one.
Anecdotal: After being out of web development for 2 years, I spent last week with jquery/mongodb/backbone/handlebars/bootstrap and feel like I never missed a beat. I feel like I gained a beat! My last project before this was on a LAMP stack, so the transition was not only easy, but also refreshing.