Trimps is a great idle game; It's got everything you need from one of these, progression, interesting choices to make, lots of unlockables with multi-tiered upgrades and challenges, time consuming activities, a simple and clean gameplay loop and a few new ideas that make it stand out from the other idle games of this world. This is currently my idle game of choice and I play it everyday, managing my Trimps to continue progressing through the game while also doing other things. It's a really neat thing.
You have plenty of different resources in Trimps; You start needing food and wood, but metal, science, gems, map fragments and helium get added after that, each of which as its own uses and methods of production. You automatically gather one specific material of your choice, and you also can hire Trimps that help you with the resource production. You hire these things, use the resources to build more structures to allow you to hire more of them and unlock upgrades to make them work more efficiently. Basic idler stuff, but you have a few choices to make compared to one-resource idle games.
Then there's the whole battle system, your Trimps have to fight through map after map of tiles with random enemies on them. You can also craft your own maps in order to find upgrades and get damage boosts for the regular world map. Your Trimps have various stats - health, attack, defense - and you can build an army of them, supply them with gear and send them to fight and get you loot, usually crafting materials, but sometimes premium currency, which can be used to buy special enemies that give you bonuses, or other temporary boosts. I think I'll be able to unlock everything just by playing the game, so that's a nice change from other free to play games I've been messing around with. After a while you get helium, which lets you buy permanent perks after you've reset the world, allowing you to do better the next time.
There are also challenges that unlock abilities and give you rewards if you complete them, they add a bit of variety to otherwise uninteresting resets and some of them really take some careful planning - or a lot of time, barring that - but it's fun to have goals to work towards in an idle game.
I think that's why I like Trimps so much right now, I still have goals to meet. Maybe someday all the challenges will be behind me and endless grind will be the only thing left, but I doubt that, since the game is updated fairly regularly. If you enjoy the odd idle game here and there, try Trimps, it's pretty good!