I have a 7 yo and 10 yo who basically are great kids, but I find myself in a lot of battles that I don't think I can walk away from. (Everyday stuff like - 'no watching tv /playing video games all day is not ok' / 'yes, it is bedtime now' / 'yes, you do need to help out picking up some of your stuff' etc.) While I'm keen to both pick my battles and provide appropriate guidance, I'd also like my kids to stay happy and I think being fun would help. Any tips?
Welcome to ParentingSE!
I'm going to try to answer your question, but it's quite broad so maybe I haven't understood what you're asking.
Children respond well to positive reinforcement. And it's nicer for the parents too. So, when they're doing something good make sure to mention it - "Hey, you're playing with your brother really nicely!" You can't praise good behaviour too much. Things like sticker charts help here.
Then, when you need to change activities present the new thing as a fun thing that they want to do, rather than telling them to stop whatever they're doing now. Turn everything into games.
Another thing that might help is letting them set a time limit. This should be short - less than 10 minutes. So, when it's time for them to go to bed ask them how much more time they need, and set a timer. This gives them some control (but not much), and gives them some warning about context changing. And, again, you can sticker chart reward good behaviour during bed time routines.
This "let them make choices" is an important tool. Sometimes it helps to offer two options - "do you want to have a bath or a shower before bed?" or "do you want me to read from this or that book?" or even "do you want to wear these or those pyjamas?" - this lets the child feel they have some control over what's going on, and it helps you guide the choices.
About chores: sometimes it's good to build a routine for a tiny chore that they do every day. So, they might leave their room untidy but they have to put dirty clothing in the washing basket, for example. Or they can leave their room untidy but they have to put the Legos back in the tub after use. Reward this with a sticker chart and plenty of praise, and increase the chores each fortnight.
But my main point is: reward good behaviour; give a lot of praise for good behaviour.
Please leave a comment if I misunderstand you.
In terms of " I'd also like my kids to stay happy..." what disturbs my kids the most is when I behave unexpectedly. Be consistent and you should be fine. My kids don't like being told off but they know when they're breaking the rules so I don't think it impacts their happiness to be disciplined - certainly when I'm making funny faces or doing the "I was right" dance they laugh along with me so being the rule-maker doesn't stop them being happy with me.
Don't let them decide when there's a battle. They're hard-wired to look for boundaries and that means that they'll always challenge you so possibly you're stepping into debates or exchanges that you're better to ignore. It's not a matter of choosing your battles - they've still got to go to bed, but you don't have to debate it with them. If my kids are watching TV and they argue when I ask them to change it, I just switch it off. If they argue about going to bed I tickle them into submission or pick them up and carry them out of the room. Find something that works for you but which sends the message that you aren't going to argue.
Try to quickly engage them with the thing you want them to do - if it's bedtime I talk about what they're going to do tomorrow as I encourage them out of the room, DanBeale's suggestion of choices is another way to get kids focussed on what you want them to do instead of why they don't want to do it. Shut down or remove anything that provides a distraction (TV, computer, pets, etc).
DanBeale's answer has good advice that's not all my style but might be right for you - my kids, for instance, are natural debaters so when I offered them choices they just debated the choices. Find something that works for you and your relationship with them. Oh yeah, and avoid my mistake of never letting them question the decision. As they get older there's more chance that they're right.