Summary (strong ideas for Black)
..h5
According to the analysis of IM Sielecki and GM L'Ami:
- play ..h5, threatening to dislodge the knight
- develop the bishop to g4 (provoking the weakening f3, gaining a tempo on the queen or pinning a knight)
- quickly strike at the center with a timely ..c5
- develop the knight to Nc6 (after striking with ..c5)
..with easy play.
Don't fear White attacks too much. In this line, it is Black who is more likely to generate initiative. Also, don't worry too much about castling - and you can also consider castling queenside (as recommended by the engine checking some lines from the mentioned repertoires).
..g6
According to IM Banzea:
- play g6, restricting the knight
- fianchetto the bishop to g7
- attack the center with ..c5 to open the bishop's diagonal immediately after the fianchetto
- take on d4, if White doesn't play dxc5
- develop the knight not to c6 (!), but to d7 and bring it to c5 or win back the pawn on c5 if White takes dxc5
IM Banzea considers the good alternatives; in his taste: ..h5 ("best, but strange" if White plays f3), ..c5 ("too concrete"), ..e6 ("tricky due to passive bishop"), ..e5 ("dull"); he considers his recommendation ..g6 to be "unusual" for the Caro-Kann due to the fianchetto, but simpler than the alternatives.
Long version:
5. Ng3 isn't a good try by White to play for an advantage. On the contrary, the engine already likes Black's position. It's almost never seen on higher levels (i.e. in the master's database). On amateur level however, it is frequently played by (up to ~20%! of) players who have semi-studied their line against the Caro-Kann and are out of theory after move 4, disregarding Black's move entirely and reacting in the same way as after 4. ..Bf5 (maybe hoping to transpose even though they're not regaining the tempo by hitting the bishop). Interestingly, players with lower lichess ratings who are less likely to study theory are also less likely to play that move!
..h5
I'd like to present 5. ..h5 (a very strong and aggressive move threatening to dislodge the knight), as it is explained / recommended in both IM Sielecki's Keep it Simple for Black and Erwin L'Ami's LTR Caro-Kann repertoires (although L'Ami also covers 4. ..Bf5).
Both claim, that play is very easy for Black, with easy development and even initiative. If White doesn't prevent ..h4, we'll play ..h4 (kicking the knight), often followed by developing the bishop to g4 which often tempts White players to weaken their structure by playing f3.
If White prevents this idea by playing 6. h4, Black can play Bg4, but even more direct is the recommended strike at the center and prepare to develop the knight to c6: 6. ..c5!?.
White can take the pawn, which leads to a situation comparable to the QGA. Black will win the pawn back (not immediately, but eventually), starting with trading queens and playing 8. ..e6 (attacking the pawn with the bishop). If White tries to hold it by means of playing b4, Black plays ..a5 and starts to rupture the queenside. It is worth looking at this line, even if it's not attractive for White objectively, Black should know how to get the pawn back with interest. White will end up with an isolated a-pawn that will be a liability rather than a strength (according to IM Sielecki).
Here is a sample line from the repertoire (not in it's entire length):
[FEN ""]
1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Nf6 5. Ng3 h5 6. h4 c5 7. Nf3 Nc6 {L'Ami gives ..Bg4} 8. Be3 cxd4 {here, Sielecki states that 8. ..Bg4 is possible, playing with a pawn down for some time, but considers his recommendation simpler} 9. Nxd4 Bg4
..g6
EDIT: I just checked Banzea's Chessable Repertoire The Caro-Kann simplified and found that he considers ..g6 to be the simplest choice for Black.
See this line in action in his YouTube video with Caro-Kann middlegame tips for beginners, as well as in his Caro-Kann speedrun (2000 Elo).
See the summary above for a description of ideas in this line for Black.
Sample line (not entire length):
[FEN ""]
1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Nf6 5. Ng3 g6 6. Nf3 Bg7 7. Bc4 c5
Just like in the line above, White cannot keep the pawn if he takes dxc5.
You should trade the queens in this case and play Nbd7 in order to win back the pawn on c5.
If White tries to defend the pawn with Be3, use your knight on f6 to attack that bishop and remove it as a defender.
White cannot play b4 to keep the pawn immediately, as this loses due to the opened long diagonal.