Some that come to mind for me are:
Sleeping.
Reading.
Drinking hot chocolate.
Reading while drinking hot chocolate.
But that’s just the basics, I’m sure.
Ooh, I love the rain.
Looking out the window watching the rain, making love, and reading. Listening to music as well. My musical taste changes when it rains, for whatever reason.
Bolding mine.
I’d probably love doing that too, but it’s kind of hard to tell without the proper props. Another person being one.
Here in Ohio, it’s a beautiful rainy day. A slight chill to the air and drizzle all day. I love this weather. It reminds me of last summer when we rented a cabin for a weekend and the weather was just like this. My DH, me, my BIL and his wife spent the afternoons sitting on the back porch reading cute little stories to one another and sitting in the hot tub drinking beer.
I love curling up on the couch with my doggies and a big blanket watching movies and napping. I also just love to sit in my car and watch the drops hit my windshield, it’s like my own little world.
The op about covers it for me. Especially if you add nibbling a homemade cinnamon roll, sipping hot chocolate and reading curled up with your favorite old quilt.
I like taking my boys outside and letting them splash around in the mud puddles. I think it is important for them to discover that it is possible to play outside even when it isn’t 70 degrees and sunny!
Besides, after a week of being holed up in the house during an extended rainstorm, the Mrs. is about ready to string them up by their toes!
I seem much more productive on rainy days. It feels so nice to be all snug and warm and dry in my office.
Snowy days, not so much. I keep looking out the window and wondering exactly how grisly the drive home is going to be.
Maturbation. Though it doesn’t actually matter if it’s raining or not.
Normally rain is a rarity here (not so much this year) and have a way of commemorating its (usually) rare appearance. Twinings Earl Grey and lemon tarts from the grocery store’s bakery. Add Enya and a good book and you are set.
If you wash your car in the rain, you don’t have to dry it, and the soft-water rinse is free. 
I used to live in a house with a huge bath tub. I would love to fill it with hot water, get some wine and read a book on rainy days.
Walking in it.
I love watching the rain. Especially when it’s really coming down hard. Monsoon season around here is always great.
I’m also fascinated by large bodies of water. Maybe that’s what I get for growing up here.
I tend to mope when it rains. Moping, though, can be fun, if you’ve got some hot coffee, a good book, and a sympathetic ear to listen to you whine about the rain.
Thunderstorms are good for sex, though.
Definitely sex. Failing that, cuddling. Otherwise, reading. And if that gets boring, Counter-Strike: Source
Max.
Well, I can have sex in any weather, so rain isn’t a big turn-on.
What I love to do is sit in the garage with the door open, listening to sports talk on the radio, drinking beer and just watching the rain. The harder the rainfall the better.
Sometimes if it’s really late I’ll do the same but sitting in the car. My wife doesn’t understand it and neither do I, but it’s relaxing to hear the rain hit the metal and watch the water cascade down the windshield in sheets.
Or it’s just the beer in both cases. Not sure.
Off-roading. Either the Jeep or the motorcycles. The earth offers great traction and the dust is eliminated. Its cooler, too.
I guess the whole sex idea would be good, too. But then the wife might find out. 
Most of the above, but espeically walking in it. With The Hat and an overcoat, I’m just like Sam Spade or Jeff Markham, walking into a bar, ordering a whisky (neat), ready to be strung out to dry by some dame and conked over the head by some thug.
Build my gallows high, baby.
Stranger
Reading about covers it for me, but a good one that hasn’t been mentioned is:
Outdoor Jacuzzi
It’s a very odd and refreshing feeling.