Depending on the time of the day you get there you should be able to spot some different companies...
First of all, there is a difference between terminal 1 and 2. Terminal 1 is for non-Garuda domestic flights only, while terminal to is used by all international companies plus all garuda flights. Visitor deck is in terminal 2.
Mornings are very calm with only Garuda traffic. In the afternoon and evening you will have a chance to see Saudia B743, Cathay A343 and A333, KLM B744, Lufhansa B744, Emirates B772, PIA A310, Air India A310, China Airlines A300 and B738, Korean A332, Air France B772, Malaysin B734 and A333, Thai A333, Qantas B747SP (!) and B763, JAL B743 and DC10, Gulf Air B763, Kuwait A343, China Southern B757, Royal Jordanian A310.
Not all of these companies operate every day into Jakarta, but you should be able to spot some of them.
Singapore airlines operates a mix of B744, A343, A310 and B777, with a flight at least every 2 hours.
Garuda operates a fleet of B744, B742, A333, DC10, B734, B735 and F28.
Some recently established local companies are also to be seem there, such as AWAIR, a new local company currently operating A310 and A300.
Generally, I agree indeed that the airport looks empty with few occasions for shopping and dining, but I am sure that, as economy is picking up, it has a great future.