Eyes, heat, water and crowds
You may look at the sun with the naked eye only during totality, and only if you are inside the path of totality. At every other moment, including at 99.99% coverage, you need certified eye protection. Damage to the retina is painless and can be permanent.
Sunglasses, several pairs of sunglasses, smoked glass, exposed photographic film, CDs, mylar food packaging, polarising filters and neutral density camera filters are all unsafe. Welding glass is only acceptable at shade 14 or darker, and most welding masks are not.
These concentrate sunlight and will destroy your eye through eclipse glasses in an instant. Any optical instrument needs a proper solar filter fitted over the front aperture, never at the eyepiece. Phone cameras are safe for the phone, though pointing a phone at the sun for two hours can damage the sensor.
Inside the path only, and only between second and third contact. The cue is the diamond ring: when the last bright bead of sunlight disappears and the corona springs out, the glasses come off. When the first bead reappears on the other side, they go straight back on. In Spain that window is between two and five minutes depending on where you are; in Egypt it is around six.
For this eclipse, heat is a more likely cause of a ruined day than anything else, and in Egypt it is a genuine medical risk.
Practical measures: carry two litres of water per person minimum in Spain and considerably more in Egypt; take shade with you rather than assuming you will find it; wear a wide brimmed hat and light long sleeves; use high factor sunscreen and reapply; eat something salty. Know the signs of heat exhaustion, which are headache, nausea, dizziness and cool clammy skin, and treat them by getting into shade and rehydrating immediately.
One quirk worth knowing: during totality the temperature drops noticeably, often by several degrees, and then climbs straight back. It is a pleasant few minutes and not a reason to relax about the two hours either side.
Children can watch an eclipse perfectly safely with adult supervision, and it is a good thing to show them. Use properly sized glasses, check that they are actually being worn correctly rather than pushed up on the forehead, and put one adult in charge of the moment the glasses come off and go back on. If a child will not keep glasses on, use indirect projection instead: a pinhole in a card, a colander, or the gaps between the leaves of a tree all project crescent images of the sun onto the ground.