There's nothing wrong with classic curl variations. However, there are ways to ramp up the intensity. Generally, there are three things you can do:
Use multiple angles
Use offset loads
Add time under tension (isometric holds or drop sets)
With these themes in mind, here are five different curl variations you probably aren't doing.
Get a bar and a band. Strap the band to the bar.
Step on the band and do 6-8 reps.
Step off the band and do 6-8 rep.
Unwrap the band from the bar and do 6-8 curls with just the band.
Find a strip of space that's approximately 20 yards, although you can play with the distance.
Do 6 regular reps using a barbell or dumbbells. On the sixth rep, hold your arms at 90 degrees and take a walk. (No space? Just do the isometric portion standing in one spot. Hold for 10-15 seconds.)
When you get to the end, do 6 more reps, hold your arms at 90 degrees, and walk back.
Using a weighted club or mace, place your hand close to the handle. Complete 6-8 reps.
Place your hand in the middle and do 6-8 reps.
Place your hand all the way at the end for 6-8 reps.
Now place your hand in the center of the club and hold your arm at a 90 degree angle. Pronate and supinate your hand for a total of 6-8 more reps.
Attach two handles to the end of a resistance band. If you have resistance bands with handles on them, just use those.
There will be four positions for this lift: hands pointed inward toward the chest, hands facing forward, hands facing slightly out, and hands pointed fully outward.
For each rep, change the angle that you're using during the curl. Once you get all the way out to the fourth type of rep, start over.
You can do this routine with dumbbells as well.
Attach a bar to a cable station and start with a reverse grip. Pick a weight you can do for 10-12 reps.
Once you do 10-12 reps, switch to a regular curl and complete as many reps as possible.