Upon arrival, be sure and check out the duty free shop.  Many Icelandic residents coming off the flights go straight to the shop to buy wines, spirits, beer by the case, and other items commonly found in duty free shops, from which one can infer it must be a pretty good deal.

The best way to get from the Keflavik airport to Reykjavik is on the flybus or by car rental. If you choose the flybus your best money saving bet is to buy your ticket in Icelandic Kronur(they also accept credit cards) and to buy a round trip ticket. It is good for anytime, and saves you about 15% of the total price.

Iceland Excursions only include transport but Reykjavik Excursions include entrance to the Blue Lagoon in the price and that makes the Reykjavik Excursions tour the cheapest way to get into the blue lagoon. 

Do note that IF Flybus list your hotel as one that they will take you to, you may well have to change buses at the BSI bus terminal on the outskirts of the city centre.  This adds somewhat to the time.   There are actually many hotels that they won't take you to, so do check this out.  Also The last bus leaves 45 mins after the last plane is due in, so if you are late, or spend too long in the very large duty free shop, you will have to take the taxi.

The airport is quite small and it only takes about 30 min to check in, suggested arrival before flight is about 1.5 hrs or a bit less, the typical 3 hour arrival is WAY too long to wait in this tiny airport. In Keflavik there is the Blue Lagoon as well, a great trip to take if you have a long layover. There are tour buses that go from the airport to the Blue Lagoon ( a 10 min ride) quite often. 

If you want to visit the Blue Lagoon for a few hours before your flight out of Iceland, you can make a price comparison here and here.

The price may look like there is a big difference but beware of what is included in the price.

Iceland Excursions only include transport but Reykjavik Excursions include entrance to the Blue Lagoon in the price and that makes the Reykjavik Excursions tour the cheapest way to get into the blue lagoon. 

Both companies offer pick-up and drop-off at all major hotels and guesthouses around Reykjavik and they are both very punctual.