Clean bathrooms a must.
Free coffee (and really good coffee) and bottled water is a big plus.
Comfy chairs and pilot lounge.
Computer connected to internet and printer (for obtaining briefing)
Fast WiFi connection for your guests. Signage posted on how to connect.
Information on how to obtain use of a courtesy car (if one is provided).
Ability to enter public area after staff has gone home. Typically there is a combo lock with the combo set to the CTAF frequency.
1) MUST have a way to get in after hours and at least use the aforementioned (clean) bathrooms. Much better if you can access the whole building except behind the front desk and the manager's office.
2) Ground transportation, preferably in the form of a courtesy car, and preferably usable after hours.
3) Information on local attractions - restaurants, hotels, golf courses, ski hills, and anything else people might want to see or do in your area. This should be on your web site as well. When you gather the info from the business owners, see if they're willing to run a shuttle service from the airport to their business. This is incredibly valuable to both the pilots and the business owners.
4) Cheap self-serve 100LL so that you actually have people using the terminal.
5) Vending machines or fridge with a money can for snacks and beverages.
6) A comfy place to sit.
7) Showers - Good both for visiting charter/corporate pilots and those who take advantage of #8 or 9:
8) Camping facilities. This doesn't need to be much, just a designated spot of grass that's out of the way off the edge of the ramp. Bonus points if there's a fire pit located an appropriate distance away from where the planes will be.
9) Other sleeping arrangements - Some airports have a couple of small rooms with a bed or two. Others have a trailer that has some food in its kitchen or a small cabin-type structure. However, if you're building a terminal, a room to sleep in for someone who's fatigued or waiting out weather is awesome. This should NOT be a full-on hotel room style thing as you should have ground transportation available to hotels for those who desire that sort of thing, and you don't want to spend a ton of time cleaning it. This is simply a place to crash for someone who didn't expect to be there, or won't be there long. A plain carpeted room with a twin bed is fine.
Where are you?