Door staff: You don’t need to tip a guy for holding the door for you, but if he runs out in the rain and catches you a cab, hand him a few dollars.
Beach towel guy: No tip is required if you just walk up to a stand and ask for a beach towel. However, if he sets you up with a chair, $2-$5 is appropriate. If there are no chairs and he goes to find one, double that.
Airport shuttle drivers: These guys are some of the lowest-paid people in hospitality, so the standard $2-$5 is appropriate. More if you’ve got a ton of luggage.
Maids: It’s a horrible cheat you’re probably guilty of at some point, but you absolutely need to tip your maids. It depends on the length of stay, but for a standard two to three nights, $5-$10 is good. If it’s longer, budget $2-$3 a day. And if you leave your room looking like a Mötley Crüe tour bus every morning, you should probably think about $10 a day.
Front desk staff: No need to tip them, though if you want your room upgraded or that embarrassing “adult feature” taken off your bill before your wife sees it, it doesn’t hurt.
Spa treatments: A 20% service charge is added to most spa treatments, but always look at your bill. If it wasn't included, 20% of the cost of the service is the norm.
Daycare: If the resort has its own daycare facility where you can drop the kids off, there's no need to tip. However, if they staff individual daycare providers who you can hire to watch your kid while you take selfies at the Louvre, $20 is appropriate.
Valet: $5 is the absolute minimum, but $10 is appropriate if you have an exceptionally nice car. And if the car has already been cooled or warmed when you come out of the hotel, with water bottles in the cup holders, $20 is good.
Amenity deliveries: Sometimes at a fancy hotel they might bring you a bottle of wine or some chips and salsa as a way of saying, “Thank you for spending half your paycheck to stay here.” They tip the delivery guys for that, so you don’t need to. If hotel staff brings you an iron or a razor or an extra bottle of shampoo, however, a couple of dollars is good.
Security: Security guards perform a lot of functions beyond just ensuring nobody steals your iPod when you jump in the pool, but you don’t need to tip them for doing their job. However, if you ask Security to grab you a towel or help you with a bag, it's appropriate to tip them whatever you would if that was their actual job.
Maintenance: If you get to your room and a bulb's burned out, you don’t need to tip when they come to replace it. But if you clog the toilet and need someone to fix it, give them $2-$5. It’ll help with the embarrassment.