No one ever plans to crash, but take it from us who have destroyed more than our share of helmets, they will save your life, or keep you from serious injury. Since you can't warranty your head, you better take advantage of it. But their real value is in protecting the palms in a crash. For mountain biking, there are full coverage gloves that protect the fingers from branches and nettles. Another purchase not to put off for later.
Riding is exercise and exercise requires rehydration.
Even a half-hour spin can be too long without water. Those first few rides for someone not used to exercise can be strenuous and uncomfortable enough without having to go thirsty. Nearly the same can be said of gym shorts, or jogging pants. No, we don't wear bike shorts just to look funny. Nothing can take the place of real bike shorts with the padded liner sewn into the seat and crotch.