You can try The Aether or the Twilight Forest. The Aether is the opposite of the nether and adds some different spawns in. The Twilight forest also adds another biome (I think) and some textures to different trees. If you want to extend the Minecraft gameplay, you can try the LotsOMobs mod and Advanced Rocketry Mod too. You can see SevTech Ages also, but you'll need a powerful pc. You can get SevTech Ages on Twitch.
Is there any Youtubers they watch and mods they have covered from them that possibly they might have an interest in? Just curious. If not then looking for mods isn't too difficult.
Otherwise can you say what version of Minecraft they are playing. 1.7.10 or 1.12.2? As they are different but the 2 biggest modded versions people play on for many reasons.
Sorry if the below is a bit of a mess, but I tried to look for whatever I could think of and from what I found along the way as I'm a tech player mostly that sometimes uses adventure/exploration but not too much anymore.
I can also recommend Datapacks which are like mods but can be added on to the Vanilla game without making downloading modloaders like Forge and doing more technical stuff a lot of effort if you don't know how. Datapacks you just download, put into the world you want that already exists and your good to go. Mods take a bit of effort but yes are more fun in the long run and have been around for years with many ideas new and old updated. I'll list some datapacks at the end. They support 1.13.2 which is the current standard version of the game players will use until 1.14 comes out.
Hmm, I'd say LightningCraft [https://minecraft.curseforge.com/projects/lightningcraft] is my go to for 'all in one' mod options (tech, adventure required to get to the dimension and gear up with armour and tools really quickly so it's not too grindy), biome mods can spice things up for exploration like 'Biomes O' Plenty' [https://minecraft.curseforge.com/projects/biomes-o-plenty] but otherwise mob mods do just the thing too. If I had to go for a good one for gearing up, a bit of multiblock structure expansion compared to the Nether Portal or just some tech like aspects, some exploration with Slime Islands to gear up your player, and very little grind in some aspects it would be 'Tinker's Construct' [https://minecraft.curseforge.com/projects/tinkers-construct].
If I had to go from a different perspective like Superhero mods or dimension mods then maybe I could recommend some of those:
Typical Heroes (seemed to cover a lot to make you a super hero compared to some that require a bit more effort with technology and so on for say an Iron Man mod which would be grindy in some ways I think)
I feel like many people get more out of making modpacks with a lot of mods they are or aren't familiar with like the ones creaseature above listed as some great choices and stand outs usually. But if I had to go with any besides the above being great choices (not counting LotsOMobs due to the download page but otherwise a great mod I always go back to when I remember what it's called after forgetting a lot about it and that it's official site was discontinued so you have to get it from 'illegal download' sites or 'mod repost' sites as the community calls them)
Have they tried any magic mods? Many can be tedious of course but some aren't too bad depending on what you can find.
While I hate these mods I think 'Lucky Blocks' is common for many in that age bracket but I'm not too sure if that's a 'thing' people are into anymore but they keep making mods like that so I assume they are still popular. I'd not count it personally since it's better with more mods or configuring them for the items that appear in them so I guess instead of I guess Vanilla with enchanted stuff or silly things in them I guess that aren't much more than a brief amount of fun or a showcase. if I had to go for a mod that does similar but you get from defeating the usual mobs or bosses in the main game that's randomised loot is 'Loot Bags'
As good as Twilight Forest but not as overhauling as RPG mods (in my opinion at least for the few adventure/dungeon mods I use/have used in the past since I'm a tech player mostly), Better Dungeons comes to mind as my favourite Dungeon mod, but it's for 1.7.10 not 1.12.2 and has sort of died but it's still really good to make the world you have in the main dimension better:
A mod possibly as good as OreSpawn which is a bit of an overkill on PCs and adds too much or might be too hard this might be a good alternative I guess for a dimension/exploration/adventure mod, I'm not sure if requires a good PC to run it:
Electroblob's Wizardry, magic, rpg, not too sure on the grind for this one but has a lot going for it to gear up players and do other things with spells from what I've witnessed. It doesn't seem as complex as Thaumcraft or Botania or Blood Magic but it seems less technical compared to those.
Modpacks based just mods can offer a lot, many do have tech or magic, or the more adventure/exploration focus or some even go for either with grinding so it's up to you what is possible to look for on that front.
Datapacks for 1.13.2 to add to the Vanilla/Standard game:
Asphodel Meadows (a datapack that makes the journey to gear up, fight what you have before that is familiar but in an effort to make the Nether 'secretly' is the case but is called Asphodel Meadows for lore reasons be changed up and be made a goal for the datapack)