Raspberry Pi's are full of possibilities, even old ones. Here is what I'd do.

It was supposed to be a pi-hole but was never able to get the dns forwarding to work on my modem.

Not sure what you mean here but there is no reason that any modem or WAN box ever really needs to involved with a pi-hole. You can set the IP to use for DNS lookups on each host by hand... OR you can turn off DHCP services on the modem run that off of the PI, which then sends the IP of the PI/PiHole for DNS as part of the DHCP lease to each client.

At any rate, ideas for it:

• PiHole with encrypted DNS service out to the internet and past your ISP's snooping modem.
• a Wireguard VPN server. This would allow for things like your phone to tunnel home use your fancy pi-hole to block all ads on your phone, privately. You'd also then have access to anything else hosted on your home network like Music/Movies/etc. Setup a samba share for that stuff somewhere. This raspberry pi can also pass your VPN client traffic back out to the internet if you setup (ip forwarding)[https://rob-ferguson.me/how-to-use-your-rpi-as-a-router/]. It's an old pi, so it won't be faster than 100Mbps, but for a phone/tablet that is likely fine.
• as a motion-activated camera or some other temperature monitoring box. You can setup a cronjob to archive the videos or send the collected temp readings to some database backend and use Grafana for a visualization front-end.
• setup a netflow collector/forwarder for your LAN using fprobe. If you network is flat and I guessing that it is, as long as you have a single network switch for both your wired and wireless clients (and a single subnet aka 192.168.0.x for all) then you can monitor the whole broadcast domain with one box. You can send/forward the captured netflow to something like Security Onion and really start to understand what's happening on your network. : )

There are so many more ideas like weather stations, news feeds, little web services for whatever.