To add to what @platypus said - on an Etrex, this works best if you're following a track (not a route. In Garmin's terminology), then at any time you can use Where To? to route to a cake shop, etc. This doesn't mess with your ride stats, the track stays visible too iirc.
By the way what RWGPS calls POI, Garmin calls Waypoints. edit: if when exporting you tick this option... Any difference anyone knows? I'm confusing everyone now...
Also, don't forget to "Reduce to 500 points" when exporting a track from RWGPS for use on an Etrex.
Supplemental smartphone use is essential, unavoidable really.
This question made me look up what Wahoo Element Bolt is capable of (noob here). If you're following a route, then for an optional detour you have cancel the current route. At least the ride stats are preserved (it's all about stats innit?).