They do change and are not in any order. They usually repeat whatever new item that they come out with as an event. Currently, that is the Guard Weapons.
To be honest, I would not do the Carnival Fever Event unless I am planning on doing that upgrade anyway such as training more troops. There is really not much to gain by doing the event. You can get what they are offering in other places. This is even truer if you have a farm.
I suggest that you save up all your building/research/guard/gems/experience upgrades for the big events like the last “New Years Event”. During those events, they offer the chance to get things that you usually have to buy like Lava Crystals, Sun Hearts, Purar Ingots, etc. When these events happen, that is the best time to use all those things that you have saved up to earn those. If you use them for the Carnival Fever before that event then you won’t have them to use during this event.
If you do not like spending money in this game (who does?), then sit on your upgrades until this event. If you do, then you should be able to double or even triple what you can upgrade vs doing them outside the event.
They run these sorts of events every two or three months, usually around a holiday. Just wait for it. You will be glad you did.
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