To not want to spend my staycation watching tv

The issue is that you want 'family time', but DH doesn't feel the same need.
The 8 year olds are dependent on you and/or DH to organise stuff.

The best you can do is organise to take the DC out, and tell DH 'we are going to X tomorrow, do you want to come?'
If he wants to come, fine, although it is annoying that he is happy to be a passenger in family life and leave the mental load to you.
If he doesn't want to come, then that is a much big problem; he doesn't value quality time with you or your DC.

Inertia will soon set in and then you find that no one wants to do very much. You're the one trying to rally the troops on your own nervously watching time slipping away. In the same boat so i understand.
Can you and your child go out, anywhere, it doesn't matter then try and plan things for a few days??

Same.

I thought op was going to say there were teens and if that was the case I was going to say they’re lucky to get them out at all! But 8 is very little, I’d want to be doing more days out etc at that age.

I also live in a similar place to OP and I’d be hard pushed to be in holiday mode here for three weeks, even though it’s full of people holidaying here. It’s just my home town and neighborouring city. There’s nothing new to do or see.

Home is ok if you have stuff planned that's different. When my two were young, one was about that age we had a long time off like you and wrote a play version of a Julia Donaldson book, made masks for each character, rehearsals and on day 6 it was the performance with an audience of their parents and Granny and Granddad.

Aren't they bored at home at 8 years old? Iii thought you were going to say mid to late teens!

None by the sounds of it.

It’s important to have a plan before the staycation starts to differentiate it from simply being at home. I would have thought there would have been a fun time with the whole family ‘brainstorming’ what do to during those three weeks, then writing a schedule up on the wall. Including chill out days of course. No point waking each morning with, ‘so what shall we do today’? You’ll just get a load of ‘dunno’s’.

Sounds like it should have been better planned, but you still have time to change things up. Family meeting perhaps about going out some days and having chill out days. I think a planned day out is better than walking out the door and just winging it, everyone will soon tire of that. Important they get to have down days though. (My son had to have the week before school started back to do nothing or it would negatively affect him).

If you only had three days off, you'd be right to be pissed off at spending it indoors. But three weeks? The odd day chilling in front of the telly won't kill you.

I think you need to make staying at home hard work, OP! Give them (including DH) jobs to do, make them prefer the idea of getting out. Obviously the odd day at home chilling is fine, but not sliding into a coma.

Can you not go and do whatever you want and they can do the same?

When ours were young we used to try and do lots of activities.... by day 4 they were begging to stay at home in PJ's.

I don't think either you or your DH is unreasonable but didn't you talk about a plan before your three weeks started?

For three weeks off and staying at home I wouldn't expect to be out every day but have a few days out (whole or part) then 1 day at home.

I think it also depends on the age of your DC. Mine at 8 and 4 so being out if preferred as at home they would end up arguing all the time and bored after 3 weeks of that.

There is a fine balance between being out and being at home, esp for such a long period.

3 weeks is a long time when you’re not actually going on holiday. You live in a tourist area yet mentioned the beach, coastal towns don’t typically have much to offer beyond the actual beach and associated activities. I wouldn’t want to go to the beach every single day for 3 weeks running. If you wanted some sort of structure you should have planned and booked activities.

So... to make you happy everyone has to not do the things they want to do?

I agree with you but tbh it was probably inevitable. That's why people go away for their holiday.

How old are the children? It would vary wildly depending on if they were 5 or 15!

Did you talk about what a three week "staycation" would look like before you both booked the time off?

We live in a tourist area with lots to do.

I'm with you in that I'd be out first thing most mornings, and wanting to do things together but I expect as you live in a tourist area your family see it as "been there done that".
Its their home town/village so probably a bit different than going away for a holiday where it's all things new and time limited in packing everything in they may want to do/see

I'd leave them to it and go out and enjoy some peace and quiet time, doing as I please for an hour or two.

I think it depends on how old the kids are but yes I’d feel the same as you. It’s good to relax but that’s not really family time, especially in different rooms. I feel that watching TV and going on the computer is what you do to relax after you’ve actually done something. I sometimes think I won’t be on my death bed thinking “I really wish I’d spent more time watching TV”, I’ll probably be thinking “I really wish I’d spent more time with my family” or “I wish I’d had more experiences” or something similar

I have two teens. We’ve done something big every 2-3 days (York, London, water park, escape room). Days out on other days may be going to Aldi! We bought a couple of new board games (Seven Wonders was a big hit, eldritch horror took a little too long for us) and binged game of thrones together. It’s been lovely.

I agree with you but, unfortunately, it means you will have to be the instigator. I would also do some advance planning - check weather the night before, ensure you have picnic supplies and bags packed so you can get out in the morning quickly and everyone knows that’s the plan. You could also see if the kids would like some one-on-one time with you - mine love this and then they can plan their day solely around them. There may be things they are more into - zip lines, paddle boarding, tree top adventures etc. Mine have to be bribed with cake and ice cream to get them to a National Trust place

I think it's good to have a mix of being at home and then going out and doing activities. I'd be broke if we did activities every day! DH and I don't usually have much time off at home together because it feels like a bit of a waste of leave (DC are primary age so it's a juggle covering the holidays) but if we, we'll usually tag team minding the DC so we can get jobs done round the house, go the gym etc.

Can you have a family meeting and plan a few days out? Can you take a solo day and do something lovely for you?