Most telescopes come with one or two eyepieces and a finder. These additions make the biggest difference, in this order.
1. A medium-power eyepiece
If your telescope came with a 25 mm eyepiece, a 10 mm (or similar) is the next step for planets and lunar craters. Quality matters more than numbers: one good eyepiece beats a box of cheap ones.
2. A Barlow lens
A 2x Barlow doubles the magnification of every eyepiece you own - two eyepieces become four magnifications. Use it on steady nights for planets.
3. A Moon filter
The Moon is dazzling through a telescope. A neutral density (Moon) filter screws into the eyepiece and makes it comfortable, especially near full Moon.
4. A red light
A red torch or a phone red-screen app keeps your night vision while you read charts or change eyepieces.
5. A phone adapter
A smartphone adapter clamps your phone to the eyepiece for Moon and planet photos. See photographing the Moon with a phone.
6. A collimation tool (reflectors)
A collimation cap or a laser collimator makes keeping a Newtonian sharp quick. See how to collimate.
Solar viewing - safety first
Never look at the Sun through a telescope, finder or binoculars without a proper solar filter covering the front of the instrument. Permanent eye damage happens instantly. Use only full-aperture solar filter film or glass made for the purpose; eyepiece "sun filters" that screw into the eyepiece can crack and must never be used. Cover or remove the finder during solar viewing.
What can wait
Colour filters, large eyepiece kits and cameras are best bought once you know what you enjoy observing.
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