Intro
As usual, don’t rely on what mapy says in terms of elevation, because for this ride it misses almost 200 m.
The route for MTB (preferable), as 50% of it is off-road and several places in the forest have sandy terrain.
It can be done at any time of the year. Mácha lake is very popular in spring and summer and mystic in winter, especially when it gets foggy and almost deserted.

The ride starts at Bělá pod Bezdězem train station, easily reachable either by train or by car (free parking is possible at area on the side of the station building).
The first 5.5 km are on asphalt following the 0060 cycling route and with a light uphill. When you reach the village Březinka keep on the same route number and (point 3) with a rocky short downhill you enter the forest.
At some point in the forest you reach what was a Baroque castle with hunting lodges (Valdštejnsko), of which very few remains today.

Continue forward, exit the forest, cross the bridge over the railway close to Bezděz train station (point 4) and start the long and slow climb until the centre of the Bezděz at the top. During this trip you don’t reach the castle, but cycle around it.
In fact at point 5 turn left to follow the so-called Mácha’s trail, marked in red, to reach Mácha lake (Máchovo jezero or Mácháč in slang) in Doksy.
The trail, lake and region take the name from the famous Czech writer and poet Karel Hynek Mácha, who walked this route from Prague several times. Today, the trail starts in Mělník.

This is a superb downhill of about 5 km completely in the forest. The terrain is just sand and roots and be careful, as it is shared with hikers.
You reach the asphalt a bit before point 6, where you then turn right to cycle on the main road for a ~1.5 km (until point 7) and then enter the forest again (point 8).
From here the loop around the lake starts and it alternates forest to asphalt.
At point 9 you cross a swamp over some wooden walkways. Sometimes the water and mud overflow and probably you will have to dismount, so check it out first.
At point 10 there is a nice view of the lake through the swamp and four tiny old and cute holiday houses made with huge barrels stand by.
Continue following the green marked path, which then connects to the blue one. Reach the view point at the tiny dock (point 12) for a great view of the lake and of Bezděz castle in the background. Then, either dismount and push the bike up for around 20 m to follow the green and blue marked trail on the ridge of the hill or take a detour by going back to the parking area to cycle on the road (check the map yourself for this second option and re-route accordingly).
Note: the section between points 13 and 14 is a passage carved in sandstone that slopes slightly downhill and it is narrow enough to let pass either a bike or a pedestrian.

Reach the Staré Splavy locality, which boasts a tiny beach, a dock, hotels, kiosks and the lake drain – its main attraction!
You could continue straight to the main road and take the red trail from there, but don’t miss the white sandy beach of Staré Splavy and Jarmila’s rock formation.
Then it is the red all the way to Doksy main beach, a nice, wide and white sandy beach.
If you are hungry and it is winter, you can try your luck at the Hawaii bar at the beach corner (it is almost always open, except for horrible weather condition). If it is summer, you can take a dip or get tanned. https://mapy.com/s/kulelugune

Unfortunately in Doksy there are not many good options for food, especially in winter. In the last year more restaurants have been opening, but in winter the choice is reduced.
If you like sweets and are up for a cappuccino, I recommend ‘Kavárna Republika’ in náměstí Republiky (Republic Square) in the centre https://mapy.com/s/kenanefuku. For any other necessities, there is a Penny supermarket open everyday. https://mapy.com/s/gamuzumene

If you are tired or it is too late, you can get jump on a train from Doksy main station.
Otherwise, follow the map and cycle back to Bělá.

Getting out of town will bring you into the sandy forest for the last time and just for a short time, because as soon as you catch the cycling path 3045 you are on fresh new asphalt. Nevertheless, this stretch is wonderful, it is just you and the nature (and hopefully few other cyclists).

At point 22 you leave the paved road to enter a field first and then another forest. There is an enjoyable flow section here, where you can practice your jumps or bunny hops. At the end you meet a car park and then you ride on the main road of the recreational area Vrchbělá. This is a sort of resort with a camping area, an hotel, a minizoo, a kids area, restaurants, an inline skating path, a rope climbing zone and more.
Finally you descend to Bělá pod Bezdězem and ride the most boring 5 km of your life through the back of the town until you reach the train station.