Enjoy some art. This is the perfect opportunity to finally go to that museum's new photography exhibit everyone is talking about so you can quit pretending you're just as cultured as your hip friend who always manages to do that stuff before you even hear of it.
Get cultured. Find a random local performance that you probably didn't even know was happening—a concert or a play or a comedy show—and snag the cheapest seats. You might discover a new favorite band, but even if it ends up being awful, at least it will be dry.
Act like kids. Think of the fun stuff your parents took you to do when you were driving them nuts inside the house. Playing at an arcade or visiting a roller-skating rink (so retro!) is a much more fun indoor activity than sulking.
Go shopping. We're not talking about those awkward mall dates you had as a teenager. Go buy something you both can do together, like fun new game, or hit up an international grocery store for a new wine to try (Lebanon and Georgia—the country, that is—have some great ones).
Check out the local booze scene. Got a craft brewery, distillery, winery, in your area? See if they have any available tours or tastings that day, or make a day of hopping between them (just have Uber be your DD).
Play grown-up games. Pool halls are basically the arcades of the older set. Being inside a dingy, dark bar on a glorious spring day would be a waste, but it's the perfect use of a cold and rainy one. If pool's not your thing, find one that offers other activities like Ping-Pong or video games while you sip from that questionable glass.
Head to a café or wine bar. Enjoy a cappuccino or a glass of red wine at a café with a view of a bustling street. It's either gloomily romantic or cheerfully hilarious as you watch people outside get soaked.
If you're willing to get wet:
Walk in the park. Braving the rain (only if there's no lightning—keep it safe!) will give you private access to a normally crowded public area.
Play photographer. Any fool with a phone can take a beautiful picture of a sunset on a clear evening (ugh, #nofilter braggers), but snap a waterproof case on and take some nature shots for the 'gram.
Video game tournament. Now's the time to dust off that old video game console you collected from your parents' house—or finally get to play two-player mode on your shiny new game.
Catch up on the classics. Stream an old movie neither of you have ever seen. Might I suggest Singing in the Rain?
Or get into something new. Are you the only one of your friends who isn't obsessed with Killing Eve yet? Me too. A rainy day is a good day to catch a few or all of the episodes of TV show you've been meaning to watch but can't seem to fit into your rotation.
Create a one-day book club. Pick a book to snuggle up with and read together, then talk about it. OK, maybe a short story if you're not a fast reader.
Build a fire. But you know, only if you have a fireplace. Otherwise it's just dangerous and not cozy.
Play board games—with a twist. What, you haven't ever played Strip Monopoly? Now's the time.
Take a bubble bath for two. Getting wet outside comes with all kinds of annoying problems, like pants that are soaked to the knee and broken umbrellas. Getting wet indoors, however, is much, much better.
Cook a luxe meal together. You have nothing else on your agenda tonight, so you both might as well try that complicated recipe you've been meaning to make someday (coq au vin is a cozy one, and more simple than it seems). Even if you're both not super handy in the kitchen, cooking together is great for bonding—and recipe fails always make for good stories, if it comes to that.
Oh, geez, just skip it all and have sex. Open the windows for some sexy sound effects and a little dampness, and get right to the best part of date night. Thanks for cutting right to the chase, rain.
