Any answer to your question will of course be slightly opinion based, but if you are more into shopping and getting some food and not necessarily into seeing historical sights (both is probably not manageable in 7 hours anyway), you don't have too many options.
I would have taken the 109 bus from Tegel to Adenauerplatz, located on the famous Kurfürstendamm avenue. The bus departs every 10 minutes and the ride takes 20 minutes. From Adenauerplatz, you can stroll 2km along the avenue towards Zoologischer Garten, passing many shops, cafés or places to eat.
When you reach the area around Zoologischer Garten, which can be considered the city centre of the former West Berlin, there are a few historical sights if you are interested e.g. the Kaiser Wilhelm Memorial Church, or if you have time, you can also continue a few hundred meters along Tauentzienstraße and visit Kaufhaus des Westens on Wittenbergplatz, one of the largest department stores in Europe or even visit the Zoo itself.
From Zoologischer Garten, you can take the X9 bus back to Tegel, which also takes 20 minutes and departs every 10 minutes.
One way tickets for public transport within the city of Berlin cost 2.70€ and allows you to transfer between different lines and interrupt your journey withing two hours after stamping the ticket. A daily pass is available for 6.90€ (is paid off if you plan to make three or more trips). Tickets can be bought from the bus driver, cash only and don't expect the bus driver to change larger bills. At the airport, you also find ticket vending machines at the bus stop, from which you can buy multiple tickets at once, which must be stamped later when you use them.
