Right in Sto. Dgo. the Hotel Santo Domingo is my favorite place. The hotel is sophisticated, serene, and definitely more on the romatic side than say Jaragua. Their guestrooms are ample and very nice without being overly done. The common areas of the hotel are limited but very peaceful. The inner courtyard offers guests a chance to relax, read or just take in the breeze. To the right of the courtyard is the large pool which is very inviting. I highly recommend this hotel.
On a smaller scale, the Hotel Palacio right in the heart of the Colonial Zone is also very nice. The rooms are done in antiques (very nice) and the roof top offer a small jacuzzi. Hotal Nader nearby is also very nice from what I have heard.
If you're willing to travel, head straight to the Samana' Peninsula. Take the Metro bus if you're not willing to drive. This is the ultimate romantic getaway spot in my opinion. Stay at El Portillo Beach Resort for a full-scale resort or one of the many smaller hotels in the area: Villa Serena, El Marinique, Club Bonito, La Tambora, Gran Bahia, Cayo Levantado, etc.
Hope this helped,
Natasha