In the park, tend to just slow down & wait to be noticed.
If someone's dawdling along the middle of a shared path - or just if someone's old, or on their own and possibly daydreaming - I give a little "ping" and say, "Passing on your right" just so they don't jump.
When four people yakking away are taking up the entire path, I just say "excuse me...excuse me...excuse me..." louder and louder, over and over, till the oblivious one (there's always an oblivious one, always furthest right!) finally gets pulled aside by their friends. Then I say a nice, cheerful thank you. They always apologise - and their friends always laugh at them.
My pet hate is oblivious parents who like to stand around gassing while the kids play, sit and even lie down on shared-use paths and cycle paths; I feel it's really irresponsible of them, teaching their kids to play around on tarmac. I "ping" the bell once and say a friendly "toot-toot!" and then thank the kids for moving aside.
A gentle ping gets their attention while the adults are still oblivious, but very small children just tend to just stand where they are and stare. I find they always seem to know what "toot-toot" means, though. Some of them quite enjoy bossing their parents out of the way.