I was a swimmer in high school; here are a few things we did that kicked my butt!
You may also want to consider getting a pull buoy - they aren't expensive. You stick it between your legs to do drills for your arms - they help you refrain from kicking as you focus on your upper body strength. If you have any noodle fragments, that could work alright.
Dolphin Kick Tread Water:
One drill that I "fondly" remember was treading water with dolphin kick - practice it without a kickboard going down the pool first.
Your biceps are locked over your ears, straight arms, hands together. Don't use a kickboard because you use your whole body to make the motion, not just your legs.
Starting flat on the water, hands in front of you, get yourself to float on your stomach. Start a "wave" motion at your hands, moving through your head/shoulders, chest, hips, and then feet. You do NOT use/bend your knees, you use your core to propel you. Your abs are doing the work, not your legs.
It takes some practice, but you'll know you've got it once you feel fluid and you can propel yourself fairly noiselessly down the pool, like a dolphin. It's also a good one to practice underwater.
Now that you're an expert, take the same motion and translate it to treading water upright - and feel your abs burn!! You can probably start out with 10 s intervals. And yes, your hands are over your head the whole time! Fun, right?
Fingertip Drag:
This is a freestyle/crawl drill for your arms.
Start by doing regular freestyle down the pool. As you take your arms out of the water, instead of reaching them way over your head, drag your fingertips along the water as you move your hand from your hip to your head. This forces your elbows up, improves your stroke, and you'll feel it work your arms/shoulders pretty well, too. Do a few laps at a time.
Sprint ladders, cardio:
Do:
2x25 [1m]
2x50 [1m 30s]
1x75 [2m 30s]
2x50 [1m 30s]
2x25 [1 m]
cooldown.
The brackets are suggested intervals: So, you would sprint 25 (one lap), and wait until the interval you have set for that lap expires. So if I set myself 1 minute intervals, then I would sprint the lap in 20s and rest for 40s. That can motivate you to swim faster, since your rest time depends on your effort! Suggested intervals are in the brackets, adjust accordingly.
Alternately, you can just rest for 30 seconds after each one.
If I didn't explain something well, let me know! And enjoy swimming; I really miss it.