While there are many things a new owner of a Dobsonian telescope might buy to enhance the enjoyment of exploring the universe, there are some things that rank higher on the scale than others.
Granted, any list would be decidedly arbitrary but I believe most experienced amateurs would endorse the following items.
To start off, I am assuming the new telescope owner has received a SkyWatcher Dobsonian with a 25mm and a 10mm eyepiece.
The first item suggested would be a Barlow lens. This is simply a lens device into which you insert your eyepiece and it effectively increases your eyepiece power. For example a 2X Barlow would turn your 25mm eyepiece into virtually a higher powered 12.5mm eyepiece, or your 10mm eyepiece into a 5mm high power lens. Barlows come in various powers but you should consider what eyepieces you have and how much increased magnification you can effectively use. A 4X Barlow would be essentially useless with your 10mm and 25mm eyepieces. This is a simple way of expanding your eyepiece collection without a significant outlay of cash.
The Polarizing Lunar Filter
The second item on my recommended list is a polarizing lunar filter. These filters come in either a fixed version which reduces the glare by a set amount, or a variable style which allows you to adjust the amount of light reduction. Reducing the light also drops the glare and often improves seeing detail. These filters can be used on Venus, Mars and Jupiter as well as the Moon. This article from Starizona effectively describes these filters.
Many observers find the brightness of the moon causes their eyes to water and create temporary light blindness as if facing a camera flash. The polarizing filter eliminates this problem and will create a softer lunar image to view.
Observing Chair or Stool
Most telescopes require the user to move up and down as the eyepiece moves according to whether the person is observing low near the horizon or directly overhead at the zenith. This sometimes results in the observer having to adopt awkward positions to either comfortably use the finderscope or the eyepiece. For a short amount of time this may not be an issue, however, if trying to locate an object by star-hopping it can place a strain on many body parts, especially the back.
Rather than trying to make use of devices to raise or lower the telescopes, the simple answer is to get an adjustable astronomy chair or stool. Besides commercially made astronomy chairs, some people use musicians chairs or stools, while others make their own astronomy chairs from plans readily available on the internet. Whichever you decide, it will be a major improvement in your comfort at the eyepiece.
No matter how careful you are, your eyepiece lens will get dirty from dust or debris from people’s eyelashes. Resist the temptation to use a Kleenex to wipe the lens as you may permanently scratch the delicate coatings on the eyepieces. Also do not blow on the lens or use your finger to wipe it. A simple photographer’s lens pen with a soft brush at one end should be sufficient to clean any residue from the lens. If the eyepiece needs more thorough cleaning, follow the directions found on the internet for safe cleaning. Here is one sample.
Make sure you have a planisphere, astronomy book, or star map; anything that helps you choose things to look at and helps you find them. That is a big sky out there and in the eyepiece it is even bigger. When you take your telescope out, it is always better to have an idea of what you want to see, and how to locate it.
A book such as Turn Left at Orion may help you identify objects you want to see, a planisphere will help you find them. Similarly a simple star map such as the freely available evening sky map from Skymaps.com monthly will list several things for your eyes, your binoculars and your telescope.
There are lots of other things that you can spend your money on which will add to your observing enjoyment, and I am sure other Dob owners will voice their opinion. However, starting out, I consider these to be the most essential.
One more thing
One final item that many people use a Telrad instead of their finderscope. Users swear by them that they make finding things in the sky so much easier. I have only used one once, but from what I have seen, I can understand why it is so popular. It also helps to save the back and neck.
I would suggest that if you buy a Telrad for your Dobsonian, get a two inch riser for it and mount it right behind where your finderscope goes. You won’t be using the finderscope when you have the Telrad on and this positions the unit at a wonderful location and height for use.
Enjoy your telescope as it opens up the universe to your eyes.