Bang-for-the-buck Parts Changes
I hate "upgrades" because anything that objectively increases the value or performance of the bike is really expensive.
But there are some cheap things you can do that will make it much more fun to ride. It's all about places you touch the bike and the bike touches the ground. So start by getting the cockpit dialed in for you. Maybe a new saddle, maybe a different stem, maybe different handlebars. I'm quite happy with an inexpensive saddle (well, inexpensive because it's old - it was sold as a racing model, ten years ago) and alloy handlebars because it's a saddle that fits my butt well and the stem and bars I use put the grips in "my" spot. I'm a clipless pedal guy, but I think clipless vs. flats is a stupid argument and you should use whatever makes you happy. But whatever it is, getting something that works well is huge. There are recommendations for both clipless and flats, at all pricepoints, all over the site. Good pedals do cost money - mine are something like $65, I think, but I have the luxury of not having to know what a lot of bike parts cost.
Good tires make a huge difference too. Go and ride your local trails, talk to local riders, and see what people like. It's not the same in every area.
After that - you can really replace everything on a bike, but it gets more and more expensive, fast. Try and ride for a season or so before you spend any serious money.