Can I enter an activity manually? My statistics are all way off.

Hej ... is there someway I can enter an activity manually?

Seriously?! Yes.

The problem is that even though I run once per week, I do Brazillian Jiu-Jitsu multiple times a week.

That isn't the problem.

Obviously I can't wear a watch doing Jiu-Jitsu. It's also an very intense workout the lasts for an hour and a half. But my watch can't track it - all it sees is a highly elevated heart-rate for half an hour after training, and a moderately elevated one for the hours that follow the training.

That depends on your watch (or Garmin device model), and what you do with it and which peripherals you use with it. For example, you could be wearing a chest strap heart rate monitor that is broadcasting over ANT+ during training, and put the watch somewhere within radio range while it is recording a timed activity using the “Other” activity profile, if you're using a Forerunner 230/235/630/735XT etc. or “Cardio” on some higher-end models in the vivo line.

So it screws up all the statistics (stress, calories burned, heart rate, etc. etc. etc.)

I can't see how either your not recording the timed activity or your post-exercise heart rate would “screw up all the statistics”.

Is there some way I can just make a "dummy" Jiu-Jitsu activity and slot it into the gaps?

Not of activity type "jujitsu", if that's what you specifically mean. However, you could create an activity of type "Other" and name it "BJJ training again" if you want, and write in the activity record how many Calories you think you burned.

Thanks, I figured out how to enter the data manually. It's a bit annoying to have to enter everything (and all guesswork) every training session. But yeah, I can do it online through the Garmin Connect website.

It screws up the statistics because it doesn't know I trained and thinks I just had a weird high heart rate.

Lastly, I doubt I could wear a chest strap during BJJ training. But maybe I guess...? Anyway, I just bought a chest strap today and it's impossible to connect it. I might have to take it back to the shop (for the second time) and demmand a refund. :(

I don't know which model of Garmin wearable device you're using, but chances are it will only connect to external HR monitors that broadcast over ANT+ (as opposed to Bluetooth).

If the device model you're using has the feature of intensity minutes tracking, then actually it should track the duration post-workout – while your HR remains elevated – as accumulating intensity minutes.