African Cichlids are pretty interesting fish. Very colorful and a lot of them would never outgrow a 26 gallon, then again, depending on the type, they can be pretty aggressive or territorial, ie taking up half the tank for itself (think sunny nest) or flat out chasing and killing other fish... but, still very good looking:
http://images.google.com/images?hl=en&q=african+cichlids&btnG=Search+Images&gbv=2
(broken link removed)
I guess the decision comes down to if you/he will want one interesting (perhaps territorial fish), 2-3 compatible and smaller size fish (like compatible A. cichlids) or a "community tank" with say 2-3 mini schools of small fish. Smaller fish (guppies aside) can be fickle, especially when bought at places like Petco, where disease and water quality are questionable at best.
You could consider smaller discus fish too, but at adult size they probably get too large for a 26:
http://images.google.com/images?gbv=2&hl=en&safe=off&q=discus+fish&btnG=Search+Images
Getting something that will outgrow the tank is probably best avoided, you'll either end up giving it back to the pet store or buying a bigger tank.
Another thing about cleaning the tank, is when cleaning the filter, do not clean it thoroughly or totally, it's where most of your good bacteria lives and thoroughly cleaning could cause you to go through a mini cycle again. Not knowing what kind of filter came with the set up, ill just go on the basis that nearly all use a sponge, a carbon bag (to start) and then some type of media (bio balls, ceramic tubes, even lava rock). When cleaning you don't ever want to thoroughly clean the top media (bio balls, ceramic rings etc) a light swish in a bucket of the used tank water is plenty. The sponge, you can squeeze out and rinse and reuse multiple times before replacing; the carbon is not worth buying, I'd use that space in the filter for more of the final media. A balanced/cycled tank does not have cloudy water or smell; the only time I ever used the carbon was after medicating a tank. By the way, while your tank is cycling (unless you use the biospira) it's Very likely that the tank will get somewhat cloudy, that's a bacteria bloom.
As someone else mentioned you may also be able to get/buy some used filter media from the pet store, whether it is some bioballs or a chunk of a sponge, if I couldn't find the bio-spira I'd try and do that, BUT only if the fish in the store are healthy and the people really know what they're doing (ie Not Petco).
Lastly, pick the liveliest fish when you do choose, even if it means making the guy chase the thing with his 3" net for 5 minutes