When picking out a snorkeling set, pay attention to the following details. Make sure the lens of the mask is made of high-quality tempered glass (not plastic) and the skirt is 100% silicone. Since it can be difficult to initially see the difference, pay attention to the product description, as the materials are always listed there.
The main difference between snorkels is the variety of additional features. If you want something light and minimalist, go for the classic J-shaped open-top model. Some other options would be to get a snorkel with a purge, a splash guard, or a dry top. A purge is often a convenient feature, as it allows you to clear the water from the snorkel if needed. Splash guards are designed to prevent small amounts of water from entering the tube when you are swimming around, while completely dry tops can close when the snorkel is fully submerged in water.
As for the fins, there are two different types for you to choose from: full foot and open heel. Full foot fins are mostly used in warmer waters. They are easy to put on and take off.
Open heel fins are a better option for colder waters, as they can be worn with neoprene boots. These can also be used for scuba diving and are generally somewhat more powerful, and durable, but also heavier. Open heel fins often come with adjustable straps, which is great if you fall between two sizes. This is also a great option for kids since they can be adjusted to account for growing feet.