There are many Linux distributions that are good alternatives to Ubuntu. This gives you more freedom of choice.
Debian Live distribution. This is a nice way to try out some Debian goodness before installing on your system.
Find some download links here: https://www.debian.org/CD/live/. Ubuntu is based on Debian anyway, so running the original distro should be a good idea.
Sabayon Linux. This is based on Gentoo and offers a binary package manager as well as the option to install many different desktop environments to customize it for your requirements. There are server editions as well as cloud images that can be used in online servers Amazon AWS style.
Download the files here: https://www.sabayon.org/.
Linux Mint. This is a distribution based on Ubuntu. This is very user-friendly and can be customized as much as Ubuntu can. The forums have been hacked in the past, probably due to poor security and not updating the forum software as often as they should. But the distribution itself is fine.
Find some downloads on the main Linux Mint page: https://linuxmint.com/.
Elementary OS. This is a very fast and sleek Linux distribution with a Macintosh OS inspired desktop environment. This is very fast and secure. This is ideal for a computer user that wants a fast desktop and does not want bloat to get in the way. it even has Appcenter, an Elementary OS version of the Macintosh OS App Store.
This is donationware, you donate a small amount of money to download this OS, but it is well worth it to run a very fast and sleek OS with a beautiful user-interface. Downloads and more information here: https://elementary.io/.
openSUSE. This Linux distribution has been around for a long time and is very secure and easy to use. I remember using a DVD version of this on an 8 Gigabyte DVD and it had a massive amount of software available. It is possible to build a custom image using KIWI. This is a great way to create a custom image for a certain user. The website is here: https://studioexpress.opensuse.org/.
Download the latest openSUSE Tumbleweed here: https://www.opensuse.org/.
Manjaro Linux. This is a Linux distribution based upon Arch Linux and is very easy to use and very fast. This would make it perfect for an older machine. The latest release uses the Xfce 4 desktop environment. This is great. Better than using bloated Gnome and KDE if you wish to have a sleek and fast user interface.
Downloads and more information here: https://manjaro.org/.
Fedora Linux. This is a very good Linux distribution, this has evolved from the very old Red Hat Linux distribution. This has quite a good heritage then. It is very reliable and has good security features like NSA Selinux to protect against Linux malware and a huge amount of packages available via the package manager. The initial release was 16 years ago, it predates Ubuntu by a long while. Red Hat Linux was first released May 13, 1995; 25 years ago. This was very good when it was Red Hat Linux 6.2, this ran very well on my old computer in the ’90s, I was amazed that it ran so well and provided so much software for free.
Download Fedora Linux here: https://getfedora.org/.