Union itself wont have much besides outdoor activities, which, in March, might be soggy. It'd be a day trip to go to Belfair or Bremerton/Silverdale/Poulsbo. It's not a really far drive, but probably at least 45-60 mins so it'd be better to just spend the day out. There are lots of parks in each area. Old Town Silverdale and downtown Poulsbo are more fun, IMO, than downtown Bremerton. But, they have just beautified downtown Bremerton (by the ferry terminal) and it's much nicer and more welcoming than it used to be. I'd pack for a little of everything weather. It might be sunny (but the ground will still be wet) or it might be rainy. But it's few and far between that it is raining hard enough to not want to go outside. It's generally overcast and sprinkles so as long as you have some waterproof shoes or hiking boots and something like a fleece with a soft shell over top you're usually good. I rarely need an actual rain coat.
Enjoy your vacation!!