The only non-obvious thing to worry about is communion. You can sit (or stand or kneel, as appropriate) through Communion in your pew, you can go up in line and just stay away from the priest, you can approach the priest with your arms crossed for a blessing, but you should not actually take the bread or the wine. Especially do not take the bread and keep it (instead of eating it); this is very strongly frowned upon.
For the sit-stand-kneel calisthenics, just look for someone who knows what they’re doing, and follow their cues. This is what most Catholics do, and when exactly you do what varies somewhat from parish to parish anyway. Should there come a point when half the church is standing and half is kneeling (I’ve seen this happen), the priest will probably give a gentle reminder which it’s supposed to be, or there might be a mention in the liturgy sheets (papers you pick up going into church which say what the readings and songs are). Alternatively, you won’t be conspicuous if you just sit through the whole thing. One might argue that it’s more courteous to do as others around you are, but nobody will take you to task for it if you don’t.
For prayers, songs, and the like, if you know that particular one, feel free to join in, but many folks stay silent: Perhaps three quarters of the church typically joins in the prayers, and less than a quarter typically sing. Given that there are sometimes subtle differences between the Catholic and Protestant versions of a prayer or song, it might be prudent to refrain, but this isn’t a matter of manners so much as of not standing out when you say a different word.
Sneaking out of the church during or after Communion is exceedingly common, for Catholics as well. I think the idea is to get out of the parking lot before everyone else, but at my church, as many people sneak out as stay, so I can’t see that there’s much point. There’s only about five minutes left to the service at this point, so you’re not missing much (mostly just the closing blessing, announcements, and the closing song), but it’s still frowned upon. Many churches also have coffee and donuts or some similar social event (which, of course, you’re completely free to attend or not as you choose) immediately following after the service, and if you leave early, you’ll miss that, too.