Riyadh is an interesting place because it is different.

There is no nightlife, but I second the idea of walking down Tahlia street, many restaurants and cafes that r decent if not good.

many good restaurants in the city, but of course no drinks.

Some restaurant suggestions:

Saudi: AL Qariya Al Najdiyya (Najd Village) on Takhassusi St (authentic Saudi food and setting on the floor, you have to try it but it is not aplce to go by yourself)
lebanese: Abdulwahab (I really think Al-reef restaurant is bad, sorry), or the more casual Al-Ajami and Ya Mal Al Sham
Italian: Cento Per Cento, L'Olimpo
Med / Itlian: Mundos at Interconti hotel
Continental: The Globe at Al Faisaliya tower, Spazio at Kingdo tower
Casual: Ninos next to Johnny Rockets on Tahlia st, Vapianos, Centro's. (you also find mumerous good places to eat and sit outside on Tahlia st if the weather is good, which is often the case this time of year). Also the food courts at Kingdom and Fasaliya towers have good offerings and it is interesting to watch the locals mingle. Good places for dinner or a snack and fresh juice on Tahlia St. with nice outdoor seating: Cafe Blanc, Waterlemon, Z-Pizza,
Indian and Chinese: many places around town, best to ask at your hotel.

Note: Restaurants are somewhat expensive (SR150-400 meal for a reasonable meal for one person), except for the more casual places like what you find on Tahlia, or when u buy local foods like kebabs and shawrmas.

If you had to pick one, go the Globe at the Fasaliya tower at sunset to get a view of the desert like city in the day time then see the lights come on at tnight. I think it is SR250 minimum spend per person. Also, note that all restaurants have a single men sectiona and "family" section" where you would be seated only if you are in the company of family or a group of friends including women.

For casual walking around and dining, stick to the Tahlia St and Olaya district if you are not familiar with the city. this is where the nice shopping and dining are located. If you want to check out the older parts of the city, you can go the Old King Abdulaziz place are and museum for a walk, or even the Al-Dir'iya district Masmak Place if you feel more adventurous. Just ask for some directions at the hotel and grab a taxi, make sure the taxi understands where you want to go before you take off, maybe have the hotel personnel explain to the driver what you want to be on the safe side and also agree on the price for the taxi ride. Running the taxi meter will cost youmore money.

DONT rent a car. If you are not used to driving in Saudi, it is very challenging. besides, road directions are not very easy and clear, rental cars are not cheap, and taxis are relatively cheap (airport to town SR60-80, within town SR10-20, unless you look like a foreigner wh does not know how much taxis cost!!!)

I ve spent much time in Riyadh and will be going back and forth a few times in the near future. Let me know if you need specific info.