There are two other types of missions that can be completed, but these aren't set in stone like Secret and Extra Stages are. These types of missions are random and can be repeated over and over, with Daily Missions giving actual rewards for doing so.
Daily Missions
Daily Missions are available once every day, of course, but you can complete them two days in advance to speed things up. If you're playing on April 30, for example, you can finish the Daily Missions for April 30, May 1, and May 2. If you do this, you'll only have the Daily Mission for May 3 available, since you already finished May 1 and May 2. These missions tend to be really, really easy to complete, and award you with XP bonuses that last half an hour. You only need to finish 10 for all related achievements, so the earliest you can earn these achievements would be about a week after playing if you've diligently looked at the list.
SOS Missions
SOS Missions appear after Green Hill - Green Hill (Stage 10) and come in three different varieties. Orange SOS Missions require you to beat the stage as the Avatar that called the SOS, green SOS Missions will have the Avatar in distress used as a Rental Avatar, and in blue missions, you use Sonic or your own Avatar to find the missing Avatar in a cage throughout the level. Dying at any point counts as a failure, and the mission will disappear. You also can't restart the mission at any time. Since the cages in blue SOS Missions vary in location, you might play through an entire level and completely miss the cage. Fortunately, the orange and green missions should be easy enough to complete, and are also nearly identical to boot. You need to finish a total of ten for the related achievements, but you need to finish 30 for the Challenge Missions, which are also related to achievements.
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