There are many recommendations, but I will try to name only the most important or the ones we liked the most:

Cave of the French: is a spectacular cave about 500 meters long with a beautiful and guided tour that you can not miss. From Aguilar de Campoo towards Burgos on the N627 indicates the detour about 8 kms, if I remember correctly.
With a sweatshirt or jacket is ideal because it is cool, about 10 degrees. Inside the cave all year round is the same temperature approx. Then you can see 1 km away, the viewpoint of Valcabado, beautiful and spectacular. And also very close, even walking on trails from one to another a few kilometers away, the Monument of Covalagua, which in summer is drier of course, but in the rainy season is spectacular.

Las Tuerces: are rock formations in which the wind and water have made very colorful shapes and some funny caves. They are also known as Ciudad Encantada (Enchanted City), because of its resemblance to the enchanted city of Cuenca. The route starts in "Villa Escusa de las Torres", to save time, although it can also be done from many other places like the Castle of Gama, etc. In Villa Escusa we park towards the end of the village, near the beginning of the route and we start. The first stretch is the steepest, but then it's all easy (for walking but without strollers or people with little mobility of course). I like to climb up to the Peña Mesa, the giant rock that has a cross on top. If you like climbing it is relatively easy to climb from behind. Very nice views all along the route. You have to try to follow the route signs to see the most beautiful.

The Horadada Canyon: spectacular views.

The Romanesque of Aguilar de Campoo and the Montaña Palentina: If you are interested, it is essential to go to the Aguilar Tourist Office, in the town square in Aguilar de Campoo, under the arcades, and get information, because there is a lot to see.

The Swamp Route: Route through the reservoirs of the Montaña Palentina to do by car in a relaxed way, and take a few baths, or spend the day in them...

The source of the Ebro River: is located in Fontibre. It goes from Barruelo in the direction of Brañosera, Salcedillo, Espinilla, and Fontibre. You can go with a pram, but there are some stairs. It is nice to see the spring and know where the Ebro is born. Nice vegetation and small route. I think it is free at the moment...
El Pantano de Aguilar de Campoo: Ideal in good weather to go to the beach area. They usually put a slide for people to bathe. This going from Aguilar to Foldada, for example, if you put it in the GPS. About 4 kilometers from Aguilar and indicates the beaches. I add that you can also put in the GPS Viarce Playa, and there you also arrive and park in that area.

The Ruesga Reservoir: Past Cervera de Pisuerga there is a very nice reservoir, with a nice route if you like to walk. It was a former campsite in 2010. It is very nice, except in dry season (which there are), you do not see the splendor of the swamp. It has many ups and downs of level during the year. You can bathe perfectly on its banks. It is adapted for bathing in the indicated places of course.
The store of Licores de la Sierra del Oso in Cervera de Pisuerga: Be careful, this store is specialized in orujos from "La Sierra del OSO", which are famous, they give you to try all of them for free, be careful how you get out of there. You are sure to get a bottle because they are very tasty.

Barcena Mayor: (Very highly recommended) It is a pretty and picturesque village, which bears a resemblance to the well-known Santillana del Mar, because all its houses are made of stone and typical Cantabrian construction, and there are various small shops of both food products (sobaos, cheese, chorizo, sausage, sausage, cheese, etc.), and curious gadgets of crafts and decoration. Well, from Barruelo it is 55km, but it takes about an hour and a bit to go quietly and enjoy the incredible landscapes and viewpoints. It goes to Espinilla, and as you get to that traffic circle, take the CA-280, and there pull map or GPS to get there and it's easy. You have to park in the parking lot enabled, because inside the village have banned the movement of vehicles to non-residents. You can walk around a lot if you want. Curiously it is in Google Street View, if you want to see it. Highly recommended to visit it. And beautiful scenery on the way there going by the CA-280 of course, not by the A67 freeway.

Cabárceno Natural Park: Well, this place is so famous, that I just write it down here for you to keep in mind. It is a mixture of Zoo and Safari, in a totally natural way with a tour in which the animals are loose in the park, usually behind their fence of course, and you get out of the car to see them. The tour is done by car inside the park, because it is very large, and you help yourself with a map that they give you. Although there are places to eat inside the park, you can take a snack or sandwich to economize your stay. It is 94 kilometers from Barruelo, or an hour and a bit. Good road.

Caves of El Soplao: They are 85 km from Barruelo, in time about an hour and a half. www.elsoplao.es I have not seen them yet, but they have been recommended to me.

La Tejeda de Tosande: 35 minutes from Barruelo. Essential rutita beautiful, circular about 10 km, it is best that you look for information on the internet. The largest yew forest in Europe. I usually leave the car in a parking lot with a wooden fence, where there is some information.

Orbaneja del Castillo (highly recommended): It takes 1 hour by car and is 66 km away, but it is worth seeing this beautiful environment through which the Ebro crosses and creates wonderful natural pools, waterfalls and canyons, depending on the time of year will be impressive or simply beautiful. To eat in any of the restaurants the best option is to go early to eat, or book, even if it is low season if it is the weekend you can stay without eating. It is not a long walk.
Cantabrian village of Argueso: Ancient village, with huts as they were built by the ancient Cantabrians, and where they explain in a guided tour how they lived, and their customs. Advisable to see along with the Castle of Argüeso which is also visitable, and very cool. They also offer guided tours.

Bison Reserve in San Cebrian de Mudá: An impressive reserve and interpretation center of the European Bison breeding, here in our area, incredible yes. Highly recommended. Call for reservations.