That leaves the sights. You need the proper Glock rear sight tool to change out your sights. Most aftermarket front sights come with a small tube of hobby grade Loctite® and the wrench necessary to fit the small bolt head that goes up into the front sight. I have XS Sights 24/7 Standard Dot sights on mine (shown photo to the right). The shallow V rear sight has a tritium "I" in the center and the front sight has a tritium center surrounded by a white circle. In daylight it's quick and easy to put the white front sight into the shallow V of the rear sight and engage your target. At night, you simply "dot the I" and can just as quickly engage identified targets. The other option I've enjoyed in sights I call "bar dot" sights. I like night sights where the front and rear sights look different. I don't like lining up dots. With bar-dot sights, the back sight has bars on either side and one at the bottom of the sight notch. The front sight is still a dot. To align the sights you put the front dot into the "square" created between the bars on either side and above the bar at the bottom of the notch. It's very easy to line up the sights and tell the difference between what is front and what is rear--even if near-pitch darkness.
I mentioned +1 magazine floorplates earlier, and I enjoy them on most of my magazines: +1 or +2s. These are an economical way to increase your per-magazine capacity and change the appearance of your weapon. On all my 9mm magazines, I have +2 floorplates and on my Glock 36 I have the +1s.
The last quick and easy change I can recommend is the addition of a laser sighting system if you'd like one. While I think the laser sights are great for diagnosing shooter deficiencies, I'm not in favor of them in operational environments except in the highly skilled hands of special operations law enforcement or military personnel. I've seen too many skilled shooters dump their basic marksmanship skills in the trash simply because they had the benefit of the magic red dot. Crimson Trace and Lasermax both make excellent units with differing features. To determine which is best for you, research them both and figure out what your needs are--then purchase accordingly.