In just 2.5 hours (as 1.5 hours will be eaten by transfers both way (15min each, and arriving back on time at the airport) there isn't that much time to see everything.

Depending on what you like and the weather, you could do maybe 2-3 things of the list:

-River cruise (included in the dayticket to ZRH)
-Walking the famous Bahnhofstrasse (From the Central Train station to the lake) - Shopping as much as your CreditCard goes.
-Taking one of the various funiculars in the city, like the Rigiblick or Polybahn
-Trying some food in a restaurant like the Zeughauskeller (a bit touristic, but not extremely overpriced)
-There are also some nice walkings through the old town of Zurich
-Taking a tram through the city to see more of it.
-Having a swim in the lake, or even better, in the river (Limmat) - there is the famous Lettenbad (http://www.badi-info.ch/Unterer_letten.html)

-Visiting the Landesmuseum (Swiss National Museum), next to Central Station, good when the weather is horrible. (Takes 2 hours)

-Going up Mount Uetliberg by train, have some food at the top, then walking down parts, then train back to city (That's just about possible in 2.5 hours without too much hurry)

So, if you really like to maximize your experience on a NICE summer day, I would do the following activities:

-Buy a day pass for Zone 10+21 at railway station airport, take any of the trains (Doesn't matter if S-Bahn or Intercity) to Zurich.
-Get off the station and go to the Bahnhofstrasse, walking along (Around 1.5km to the lake, last 500m you could also take the tram from Paradeplatz to Bürkliplatz), do windowshopping along Bahnhofstrasse. At Paradeplatz, the famous Sprüngli shop/cafe is worth a visit. If you're hungry, the Zeughauskeller is also next there.
-At Bürkliplatz, catch the river boat to Station Landesmuseum/HB (back on the Limmat river, HB means central trainstation)
-Walk through Platzspitz park, then over the passenger bridge over the Limmat river, then on the right side of the river along the Limmat, to the riverbath "Unterer Letten"
-Have a relaxing swim in the river (Entrance is free, fyi)
-Take tram back to the mainstation (4 stations)

That would be doable in 2.5 hours.. if you don't like to swim, you could alternatively visit the Altstadt (old town) by a walk, but nowadays it's 20 Kebab stalls and 50 international brand cloth stores..

Costs: Ticket for all transport combined is around 12 CHF