Perhaps the best thing to do here is look at the interactions you can set up for your players. Here are three basic ones which many consider the bread-and-butter for most DnD adventures.
- Combat: go and beat them up to achieve your goal.
- Puzzles: figure out this puzzle. You need a motivation to solve this, and it can take a while.
- Social Events: deal with NPCs. Bluff, intimidate, deceive, and negotiate your way to reach your goal.
It is important to note that different groups of players will enjoy these three things in differing amounts. DnD tends to be a combat-heavy game, so being cognizant of the other interactions and general player-engagement is important. Breaking up the monotony of combat with these other interactions is a really good idea. Changing the fundamental interaction with the game can reign players back into the game.
So, to your question, I would actually recommend non-combat encounters next time they get distracted. It is a change of pace, and you and your players may find it quite refreshing. This is especially true in DnD, because it tends to focus more on combat than other systems do.
Here are some ideas:
A wandering merchant who is lost in otherwise hostile territory. Direct or guard the merchant out to a safer place.
A wounded ranger, who really needs some healing, is happened upon by the players. Magic healing and medicine checks carry the day here.
Some too-powerful-to-fight people are considering the life of a bandit, but they really don't want to fight, and the players could "talk sense into them" without lowering their HP. Too-powerful-to-fight people can be things like an assassin NPC.
A wizard needs them to find some herbs or some items the wizard cannot normally acquire, or this wizard wants to gather reagents and enlists the help of the characters. This is especially true if the wizard is not proficient in nature and survival skills.
A fey creature (such as a pixie) wants to challenge the characters to a game of riddles, tongue-twisters in sylvan, or a headstand-contest, which isn't fair considering pixies fly.
If you're still set on combat, I recommend something tough, like an owl-bear or a gargoyle. Make sure it's just 1, and if you still think it's too much, you may lower it's HP. Altering HP has always been an easy way to scale encounters. (The other is altering the number of whatever they're fighting.) You can throw hobgoblins in your usual goblin mob mix to give the encounter more lethality. Depending on your players, you can justify a lot of things showing up.