I had the chance of taking a much needed break from WDW and hitting Universal last year in mid-December. It was raining when we woke up that morning, but we anticipated that it would go away once we hit the park. It didn't. My mom and I found this particularly odd since hurricane season ended just a couple weeks ago!
Well let me tell you, I wasn't that disappointed once I saw the line at the gate. I think there was a total of six families entering IOA!! We didn't arrive that early either (around 10 or 11am). It was like this for almost the entire day. The rain wasn't that heavy (though non-stop), but it was mild enough to keep both parks nearly empty.
In my opinion, all the rides are just as enjoyable as they are if it were sunny and clear. The only exceptions being probably Hulk and Dueling Dragons. The rain whips you pretty hard in the face should you choose to sit in the front!

Less so if in the middle or back. Please also note that my mom and I choose mostly the "scary"/thrill rides. The mildest we went on was probably Jurassic Park River Adventure. It was quite fun riding a wet ride when you're already wet! My mom couldn't share the sentiment and wanted to get off the second her butt touched the wet seats.
The only ride we couldn't go on was Hollywood Rip Ride Rockit since it was an outside attraction at pretty high heights. Pteranodon Flyers was probably closed too, but we had no interest in a "kiddie" ride. The rest of the rides stayed open.
In conclusion, it's all up to you on whether you want to have fun or not on a rainy day. I made the best of it and had an absolute blast! And we did do everything
we wanted to do in one day at both parks. We went on a weekday too, so choosing Thursday is probably a good idea on your part.
Hope my advice helped!
