According to an article in La Nouvelle Gazette:
In 2003:
1. prince Philippe
2. king Albert II
3. princess Mathilde
(2002's ranking was:1.Philippe,2.Mathilde,3.Astrid !)
It should be mentioned that the number of activities is that of the public visits, so not including the hours spent in their offices, or all the audiences.
PRINCE PHILIPPE: 87 activities
The most active member of the royal family. The eldest son of the king actively prepares to take over his father's role. His office is at the royal palace, just like his "house", that coordinates his (and his wife's) agenda . He also receives a great deal of people in audience at the palace, who keep him informed of the current news in all Belgian sectors.
KING ALBERT II: 61 activities
Receiving people at the palace, visiting firms, expositions, gala nights, representing Belgium abroad, presence at sports manifestations (e.g. Tour de France), taking of the oath of the members of governments. To this, and many other activities should be added more than 90 days of audience (3 per day, and one for the prime minister every week), and his daily work that only the king and his staff (4 close counsellers) can mesure.
PRINCESS MATHILDE: 56 activities
She combines her role of princess with that of mother. She goes to her office at the royal palace at 9h00 and returns home at 16h00. That's the minimum program. She also undertakes solo activities or with her husband. Visits in Belgium, economic and humanitarian missions abroad, work for Unicef.
PRINCESS ASTRID: 54 activities
Because of an operation she has been "out" for 3 months. Her activities mainly focus on the medical sector (she's president of the Belgian red cross).
QUEEN PAOLA
Very rarely seen on a visit without her husband. She's less present on the royal scene than her children. No number of activities mentioned.
QUEEN FABIOLA: 21 activities
Queen Fabiola, now 76 years old, is rarely seen in public. When she's present, it's often in the company of the rest of the family, or to honour the queen Elisabeth Competition with her presence. (she's the honorary president)
PRINCESS CLAIRE & PRINCE LORENZ: 15 & 14 activities.
Lorenz works full time as a banker, Claire half time as a surveyer. SOmetimes they accompany their spouse on an activity, and they're usually present when the entire family in united.
PRINCE LAURENT
A big question mark. His foundation (for the well fare of animals) refused to release figures on his activities.