I'd buy a purpose designed .22, many examples cited above, rather than a .22 that either mimics another design, such as the GSG, or one that is a conversion or trainer for one (M&P, Sig...).
Too many compromises in those, be it the lousy M&P trigger or the pot metal construction of the GSG.
My advice is to buy used, and an older model. The Ruger MK-II is all steel, has no goofy magazine safety or ugly loaded chamber flag. You can get them for $300-$350.
Revolvers... The only one that I know personally is the S&W model 17, and you're going to have to be both patient and picky to find a very nice one within your budget.
There's Alfa and Ruger as well, but I've never tried either and, having a Smith, I'm never going to.
.22 can be lots of fun. Shoot paper to develop skills then get into reactive targets - cans, plates, etc... I've come back to both .22 and .38 special after years of thinking they were too boring (when you're young you want boom). It's how you approach something that's going to keep it interesting for you.
Heck - I bought some sub sonics and had a blast with a gun that was now more quiet than an air pistol.
I hang pop cans from the target returns with a string - you need some skill to make them dance, since it requires a hit near the top or bottom, where there's more metal to punch through. Or try to shoot the string to bring the whole thing down.
If you find .22 boring, you're approaching it with the wrong mentality - the boom mentality.
Boom is great too, but there's more out there than boom.