Are 7 hours enough time to leave the Brussels airport and go to the city center?

It is a 20 minutes train ride to the city center. A single ticket costs 7.80 EUR and a return ticket twice that price. There are at least 3 or 4 trains per hour. Details about train travel can be found on http://www.belgianrail.be On that site you can also buy an online ticket. This will save you some precious minutes. However, you should be aware of the fact that these tickets are not refundable in case you are not using them.

You should have enough time to go to the city center. The best is to step out of the train at the Bruxelles-Central station. It is just a stone's throw from the famous Grand-Place. In that area there are plenty of spots for having a good beer and having lunch or a snack. Here is a nice spot, just next to the Manneken Pis: http://www.poechenellekelder.be/

There is an alternative to the train. The bus lines 12 et 21 link the airport to the European district. You have to get out at "Schuman" and proceed from there by metro (line 1 or line 5) to the city center. It is cheaper than the train (4 EUR for a single ticket), but takes longer (30-35 minutes in the most advantageous setting). However, the bus can be an interesting alternative if the railway employees are on strike (happens at least once or twice each year).

There are indeed luggage lockers at the airport: http://www.brusselsairport.be/en/passngr/services/luggage_lockers/