Plastic pedals with metal pins like Raceface Chesters or similar would be sensible, and checking to see if the bike needs any maintenance (worn chain, etc). Other than that it seems to meet your needs, so why mess with it?
Upgrades to working parts are usually a poor use of money unless you have a pretty concrete complaint you want to resolve. For what it's worth, pretty much the only other thing that comes to mind that could sensibly improve the bike would be high end, high TPI tires with tread well suited to your riding terrain (sounds like you could probably get away with relatively low profile tread), although for best value you can wait until yours actually need replacing.