I decided to shift my “check social media” reflex to “do a bit of language learning” and it worked great for learning hiragana and katakana - I was using an app and would just do a quick drill that took a minute or two and after a couple of weeks I had learned it pretty well.
When I have wanikani reviews or lessons they fill the gap perfectly, but sometimes I’ve already done my reviews and have a bit of time to fill.
what do you do in these gaps?
My life has changed very positively since I’ve started implementing this. Doing nothing is underrated. Being in a rested state of mind makes studying 10 times more efficient.
Seconding that downtime can be underrated for sure.
That said, digging out my old portable consoles has been wonderful for me squeezing in a little reading when I have the chance. I used to wait until I had good blocks of time to get started but right now I’m reading Robotics;Notes Elite on the Vita and I can have that woken up from sleep and read a few lines in an instant; it’s nice. I find visual novels like this with all the immediate contextual reminders easier to do in tiny blocks like that than normal books, which I like to still reserve some time to properly get into.
I’ve done the same in the past. Usually jpdb.io reviews (I use that instead of Anki)
I have the WaniKani Sentence Practice Tool on my phone. I can read a few sentences from WaniKani’s example senteces using kanji that will be coming up for review in the next few days, or week, or however I want to set it.
I work from home teaching business English, and I enter the online classroom early. Some of my students are also often late for class. So, if my reviews are done, I tend to read manga on my tablet or something like NHK Easy News. This is how I finished the last book club manga ahead of schedule 
If I know I have 10 to 15 minutes, I might try to work on my Japanese textook exercises, but I take a bit more time for these and need to write, so they’re not as effective for 5-minute breaks.
Either start reading manga or skimming, to build desire to read. Or perhaps actually do nothing (and browse the forum). It depends on what I am standing by for.
AnkiDroid handwriting deck, maybe, but WaniKani is generally a serious business and takes time.
A quick grammar reference might be read though, but let’s not hope much.