My latest SOP video addresses out of town travel.
For most of us, out of town travel is a quick day trip. From time to time we also have longer, overnight travel.
For all travel, you need a consistent policy about mileage, meals, and public transit. There are some guidelines from the IRS (or whatever tax agency you report to). Start with those as a good guideline.
For overnight travel, things are just a little more complicated. You need to have a policy that includes air travel, car rentals, hotels, and the "class" acceptable for each of these. We encourage basic comfort.
If employees want something more luxurious, we let them take care of that. We also let employees keep the reward points for any of this.
Whatever you do for your travel policies, you should remember to keep it simple - and do it consistently!