I used to wait tables so I'm generally a "start at 20%" tipper for table service and generally at least throw the change in the counter cup. If I'm in the drive through at Dunkin or something and my bill is 6.50, I'll tell her to just give back 2 bucks and throw the rest in the jar.
Some places will automatically include a tip in the check if you ask them to (or automatically if you're in a large party). If you don't want to do maths, just ask if they can auto do that. Often it's 18% in my experience.
As for hotels, I generally leave 1 or 2 dollars a day when I stay (my company allows me to expense up to 2 dollars daily for hotel maid tips). I place it hanging over the alarm clock. If they don't take it, I'll leave a note saying, "this is for you" or something along those lines.
Cabs, I usually at least round it up if not give a 10% or more tip, especially if the ride was good. If I farted in his cab, I'll tip higher :rofl1:
Other things you may not experience here is when someone loads your car for you (like when I get stuff at the garden supply store, etc.) I will generally offer them a few bucks. They don't always take it, which is fine. I have done that job as well in my youth and always appreciated when people would throw me a buck or two. Paid for my lunch, usually.
So, a guideline to use, if someone is providing a service for you and there is a fee involved (table service at a restaurant, cab ride, hair cut) offer a tip based on a simple percentage of the fee you are paying. If someone is providing a service for you that doesn't generally have a fee attached (such as maid service at a hotel, carrying bags, loading your car), then a couple of bucks is fairly standard.
Everyone is on a budget, especially when travelling abroad, so don't kill yourself over it.