The idea of a sandbox campaign is to let the players set the course of the campaign. If they decide to go after Smaug the terrible at first level then so be it.
The referee's creativity comes into play not in prepared content but in deciding and developing the consequences of the player's actions. Because mostly likely a group is playing a game for entertainment and not a true simulation. The referee doesn't have to always pick the most probable consequence but rather look at the list of likely consequences and pick the most interesting that leads to more opportunities for adventure.
For example you gave two situations.
We know from reading the Monster Manual that Dire Boars are ill-tempered animals that are dangerous. Players that had considerable success in killing large numbers of Dire Boars will likely earn the gratitude and respect of the surrounding villages. So when the elders of Eastwick ran into trouble with humaniods operating out of a ruined castle near their village, they sent a messenger to the party asking them for help and offering a sizable reward as well as some land.
Despite being an important party of the region's background the players ignored the fact they were hunting in a Royal Forest protected the King's Rangers. Now several months the Rangers discovered who was responsible but luckily due to some friends the party was tipped off and fled. Now wanted fugitives they will have to be careful for the next couple of session until they are able to reach and cross over the kingdom's borders.
Bandits Robbing Humaniods
The Sheriff of Eastmarch has had enough of the increase number of successful robberies on the Royal highways. He has authorized Sir Fenwick of Appledore, a Knight of Renown, to hunt down and bring the brigands to justice. The entire march is abuzz about it and Badger, the party's fence, warns them at their next meeting.
ShadowCat the leader of the Company of Honorable Men, the thieves guild of Castle Blackmarsh, has heard of the success of a new group of brigands. He sends Luven Lightfingers out to fine them and to extend an offer for them to join the Honorable Men. In return for a small tithe, he will guaranteeing their territory as well as put them in contact with those of Blackmarsh's merchant willing to invest their money with no questions asked. However if refused he will bring the full weight of the Honorable Men down on the party.
All of these possible consequences stem naturally from the prior actions of the characters. Developed in a way that gives them further opportunities for adventure. While being hunted doesn't sound like a reward it is an adventure and if the challenge is overcome then it will lead to other types of opportunities.