Play sand is a fine substrate, gravel, aquarium planted substrates... whatever you prefer! Each has pros and cons and can affect what you plan to stock the tank with.
If you are looking for bottom dwellers like pygmy species of corydoras (pygmaeus, habrosus, or hastatus) then you will need a sand substrate.
If not, anything you want is fine.
There are many nano fish, but it also depends on your water hardness. If your water is over 215ppm GH, I do not recommend sparkling gourami, but if its below they will thrive. Higher GH has a higher chance of them dying from various issues--columnaris has proven to be more prevalent in softwater fish when kept in hard water. And sparklers are already sensitive to water quality.
This goes for any soft water fish kept in hard water, a good source to check fish GH requirements is
www.seriouslyfish.com just search the species you are interested in and it will tell you the GH range either in DH or ppm. You can convert the two with water hardness converter sites too to figure out what DH=ppm.
GH refers to your hardness, not pH btw. You can either use a liquid test to test your GH or can look for it on your water provider's water quality report, often listed under "total hardness"
Opposite issue is keeping hardwater fish in soft water, they become mineral deficient and often are prone to succumbing to fungal infections easier, and do not thrive. Common issues are shimmies seen in livebearers as well.
Guppies can handle as low as 145ppm GH, the least killifish a bit lower, but most need 250ppm and higher.
If your water is over 250ppm, you can look into endler guppies who need hard water. Perhaps some of the nano rainbows...
If under 215, sparkling gouramis, pygmy cories. Ember tetras. Microdevario kubotai...
If under 170ppm maybe look into some of the microrasboras or boraras like chili rasboras...
A regular domestic betta can handle up to 300ppm GH, so they would be good for either hard or soft, but many of the domestics are weaker genetically nowadays too, so you may have mixed success with them staying healthy unfortunately.
Plant suggestions, crypts do nice, there's many small species you can use for a 10g (parva can be used as a carpet even!).
Hydrocotyle tripartita is nice for small tanks.
Java fern, anubias, mosses... all easy low light low tech plants.
I would steer clear of most swords outside of chain swords due to the sheer potential size of them. Ive had some echinodorus bleheri reach 24" tall.
An interesting source backing up my info if anyone wants to learn more