Most the great recommendations have already been said, but I'll comment anyways as 2D platforms and metroidvania's are some of my all time favorites,
I agree that Steamworld Dig 1&2 are both amazing, very chill feel overall. The second game is a direct sequel and better put together, but the first is still worth playing.
VVVVVV is just plain fun (and cheap). Main game is fairly straightforward, only getting all the extras is difficult and it has a fantastic sound track to boot.
While I really enjoyed Bloodstained and would say it's well worth anyone's time, it does require learning boss patterns and likely some grinding for it's RPG mechanics.
The RPG style Wonderboy games are some of my most replayed games. Very well put together. Used to be able to get all the earlier ones on the Wii Virtual Console. I believe there are only a few for PC digital sale though. Wonderboy 3:The Dragon's Trap (GOG/Steam) [One of my all time favorites], Monster boy and the cursed Kingdom (GOG/Steam) [Fantastic fan Game done with the original developers blessing], Wonderyboy in Monsterworld(Steam-Genesis Emulation).
On the topic of the Sega Genesis, there were many good 2D platforms for that system and you can pick up a good variety for cheap either on steam (The 2D Sonic Games which I believe that Jazz Jackrabbit was trying to compete with) or on the genesis mini (Including Monster World IV which is another in the Wonderboy series but with a female protagonist, Genesis mini only).
Treasure Adventure Game is indeed a fun free game, the developer also finished a remake of it a bit ago called Treasure Adventure World which runs better and is more polished overall,
The old Apogee games I definitely have nostalgia for, but they have aged quite a bit. For example many of the games use the old PC speaker (Beeps and Boops). They usually have light to medium platforming with light puzzle solving involved. Fun games, and I still enjoy Secret Agent.
Oh and if she does enjoy Puzzle Platforming you can get Lost Vikings and Blackthorne for free off the Blizzard site
https://www.blizzard.com/en-us/download?show=classic . The Trine Series here on GOG is kinda in the same genre as the Lost Vikings but much newer,
Hmmm, what else. Oh Disney's remakes of both DuckTales and Castle Of Illusion (Steam) are well worth playing, though check the System requirements. The Disney Classic Game collection of Aladdin and Lion King is also here on GOG, but I haven't personally played it.
....Just noticed you also asked for some great adventure games too. I'll keep this list shorter.
Indiana Jones and the Fate of Atlantis (Seriously a must play)
Broken Sword (The first is especially a classic and the newer edition shows all the clickable hot spots for easier play)
Heroine's Quest (Free on Steam, A great hybrid RPG/Adventure game based on Norse mythology. Get it, it's amazing)