It's kind of amazing how cheap things are these days (although quality is often questionable). In the last few months I put in a GPS driven "speedometer", solar panel and voltmeter for about $60 total.
The speedometer reads in mph and has a variety of settings, including an overspeed alarm (don't think I'll be needing that!). It's got a very large readout and is reasonably bright in the sun. In a car test it seemed to read accurately at Kelt speeds. Locks on to all needed satellites in a few minutes. Kinbelle Universal Digital Car GPS Speedometer Speed Display. $25.
The solar panel is far better quality than the older unit I had. OYMSAE 10W Solar Car Battery Charger Portable 12V Trickle Battery Charger. $32. Haven't quite figured out where to mount it. For now it's on my binnacle. Above it in the photo is a solar security light from Harbor Freight. Also pretty cheap and surprisingly functional.
The digital voltmeter replaced the old analog gauge. A pack of two was about $14. 2 Pieces DC 12-24 V Car Voltage Gauge LED Display Waterproof Voltmeter Digital Round Shiny Panel Voltmeter .
Two years ago I replaced my analog voltmeter with a "multimeter". You can get them for $20 US (shunt included). I find it very useful for checking current draw and it hardly uses any power on its own ( <5 mA). One thing it doesn't do is measure current in both directions (if you generate more than you draw, it reads 0 Amps). Now it looks like you can get them with battery monitor features for just a few dollars more.