I've a few days off work and I'm trying to pack as much japanese study as I can for three days (today, tomorrow and Saturday). I'm about 500 kanji in rtk but I'm not sure which subject I should focus this marathon on. RTK, grammar or Core? I can understand some japanese but I'm really bad at output (writing or talking). But taking RTK out of the way is something that I want to do for a while.

Also if anyone is interested in joining I think it'd be great

The trouble if you're using an SRS program (which you should be doing!) is that once work starts back up you'll be saddled with a ton of reviews which you may not have time for. Whatever you decide on, remember to keep it in moderation or you'll hate yourself for it later.

How many RTK frames do you usually do in a day?
You could try to double that by doing your routine once in the morning and once again in the afternoon. Or triple it (morning, after lunch, evening), but you shouldn't do a lot more than that or it will be a pain to keep what you've got once works starts again.

The rest of the time you could do some vocab/grammar and maybe some reading of your level? Possible something you enjoy, so you don't get back to work more tired than when you left.

At least, that's what I'd do if I had a few days off

I might follow your advice. I could add a lot of kanji to my "learned list" but when I get back to work I know I won't be able to review as much as I did these days.
During the week I usually only do reviews (the count depends on my time) and only on weekends I add more kanji to my list. Finishing RTK is definitely slower this way but right now it's the only thing I can manage.
I'm going to add a few more kanji and in between read some japanese manga and watch some kind of show or movie with japanese subtitles.
Thank you for the advice

Then treat every day as if it was a weekend: add kanji every day.
You said you have "a few day". How many are them? If they're enough, you could try to pack more kanji learning at the beginning and slow the pace 2-3 days before getting back to work. This way at least the first wave of expired kanji will be less hard when you get back.

Thursday I added only 5 because some of my most recent kanji were forgotten and I didn't want to add more unless I knew the previous ones. Yesterday no japanese study and I'm today I'm aiming at 50-100 new kanji depending on how well it goes.