Stars are a collectible that increase the countdown timer, preventing Baby Mario from being taken away from Yoshi as soon if hit. Stars can be found in winged clouds (three or five stars depending on the cloud), tulips (eight stars) and two stars can also be spawned if Yoshi hits an enemy or other interactive sprite with a red egg. Stars are collected on contact and can also be tongued by Yoshi. There is theoretically no limit to the amount of stars available in a level. In 100% speedruns, you must collect 30 stars in total, which gives 30 of the 100 points needed. In Any% speedruns, stars are available as a "safety net", but are usually ignored.
Red coins are disguised as normal, yellow ones. However, while they are disguised, they have a slightly red tint. (Some normal coins have a red tint as well). They are also slightly larger than the yellow counterpart, allowing for easier collection. There are 20 red coins in each level of the game. In 100% speedruns, they all must be collected. This gives you 20 of the 100 points needed. In Any% speedruns, it is best to avoid these, as their listing on the score screen takes a few frames.
*3-E has 21 red coins. It's most likely the one hidden in a post underneath a tap-tap.
Flowers are a collectible within Yoshi's Island, with five in each level. They can be collected by touch or by contact with a thrown egg or spat out enemy. In 100% speedruns, all five flowers in a level must be collected. This gives you 50 of the 100 points needed on the end of level score screen, plus an extra life. In Any% speedruns, you should avoid flowers because they can trigger bonus challenges as well as add extra time to the score screen.
*5-7 has six flowers. The sixth one is hidden off-screen but can still be collected with an egg. This is used in 100% speedruns to skip a much slower flower.