You could do a combination of daily challenges and achievements.
I saw some mention of "milestones", but I'm quite new to this game (and never played GMT), so I'm not sure what's planned/already in the game.
However, I have played many other games in the past that use some sort of system that rewards players for playing a lot, either based on time, based on achievements, or a combination of the two.
The one game that comes to mind is Guild Wars 2. You get achievements for doing just about anything. Logging in, doing a list of daily objectives (which would be akin to the daily challenges proposed here), or doing actual content in the game.
I think we have to be careful, though, because Tower Unite is a social game, and you want to avoid making it a game where people just log in, do daily objectives, and log out because they've checked off a list.
So, you could offer points for doing various things in the game, along with Units.
First time playing a certain minigame? Get achv points. First time doing a minigame for the day? Earn points.
Played for 10 hours? Earn some points.
Whenever you reach certain point 'milestones', like 500 points, 1k points, etc, you unlock some kind of exclusive item and/or bonus.
Maybe at 1000 points, the object limit of your condo is increased (if thats even a thing - if not, dont make it so lol)
Or, maybe you get a voucher for 1 item from any store for units.
Also, skins, face features, accessories etc. Exclusive ones for reaching milestones.
I think whatever system you implement, it has to encourage people to actually stick around and have fun. Don't reward people for just logging in, checking off a list, and logging off. If I see a player with a certain exclusive item, I have to think "oh cool, they must be a long timer here"
I'm not sure how important these systems are to the game though, given what type of game it is.