wow, a cube aquarium, very good choice for a betta :)
Some easy aquarium plants

ceratophyllum demersum (hornwort)

it's an excellent plant that absorbs many pollutants. It can live very well without being rooted and therefore an excellent float. It should be pruned often because it is weed.
In the first six months or so during which your aquarium will mature, the presence of this plant will help you greatly to avoid algal and bacterial infestations.

anubias nana, anubias barteri

slow growing plant with excellent shades of green. Ideally it would be best to place it tied together with a mangrove trunk. It grows very well in areas of semi-shade

agenandra meeboldii

For a touch of brown red colour in aquarium.

cryptocoryne lucens

very elegant green plant of beautiful appearance with thin and long leaves up to 20cm tall

info

A fertile bottom in aquarium will greatly help the growth and maintenance of plants. The CO2 system would be an additional aid, but it is not required for these types of plants.

all these plants require basic weekly fertilization