Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
A seamless pattern formed from a square tile based on a jpg on Pixabay. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Almost like little fish shells, or dragon skin.
Source Graphiste
Prismatic Geometric Pattern Background 2
Source GDJ
From a drawing in 'Jardyne's Wife', Charles Wills, 1891.
Source Firkin
You could get a bit dizzy from this one, but it might come in handy.
Source Dertig Media
Prismatic Chevrons Pattern 5 With Background
Source GDJ
It’s okay to be square! A nice light gray pattern with random squares.
Source Waseem Dahman
Here's a camo print with more tan and less green, such as might be used in a desert scenario. This is tileable, so it can be used as a wallpaper or background.
Source Eady
Prismatic Abstract Geometric Background 4
Source GDJ
Nasty or not, it’s a nice pattern that tiles. Like they all do.
Source Badhon Ebrahim
From a drawing in 'Hundert Jahre in Wort und Bild', S. Stefan, 1899.
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Number 3 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
A free seamless background with pink spots.
Source V. Hartikainen
An abstract texture of water. It's not perfect, but will do. You may download if you like it.
Source V. Hartikainen