Stuff to put into homelab rack

I am a bit bothered about my rack being (more than) half-empty and I am always wondering what else could I put in it, so I am interested in your ideas for installation.

What I already have:

• 2 servers with 3U in total, at the moment I don't need more
• shelf for ISP router and OPNsense mini PC router
• switch (OFC)
• patch panel
• cable passtrough brush thing (not sure how it is called)
• a pull out shelf for placing my notebook on it when configuring stuff
• 2 regular shelves that I keep some cables and tools on

What I am missing/planning to get to fill in some of the missing 20Us:

• rack UPS - currently have a tower one with a perfectly sufficient capacity, but of course i'd like a rackable one much more, just looking for a good deal
• KVM console - i don't strictly need it (my servers have IPMI), but seems as a nice addition. unfortunately even fairly ancient ones look expensive, often missing proprietaty cables, etc.
• ....what else? staff like ventilation units does not look useful for me.