Well, with just under 4 full days it stands to reason hopping to another island won't leave you with as much time. Figure a few hours' time on one day just making the trip to the other island (factoring in getting to the airport, rental cars at both ends, wait time, checking in and out of hotels, etc). But it's your vacation so only you can decide.

One can certainly spend more than 4 days and not even begin to appreciate Kauai. Is it enough time for you? Don't know. What do you want out of the trip? What sorts of things do you and your family like to do??...active vacation, beach bumming, nature, cultural, historical sights, fine dining, etc, etc??

Has the OP investigated hotel availability and prices? Last minute rates for a winter long holiday weekend (Monday is MLK federal holiday, generally observed by all governments, schools, and banks but not private employers). Hawaii could be much more expensive than anticipated, if one can even find a room for this period (and switching hotels every night would be miserable).

Excellent advice. I haven't booked the hotels yet. Any suggestions for some really good hotels?

I have about 124k Club Carlson points, 30k Marriott points, 18k IHG points, 42,000 Amex MR.

Also have 100k AA miles balance left so I can use those on hotels if I'm getting a really good value?

P.S: Also have $400 of rewards in Barclay Arrival Plus card.

Check TripAdvisor for reviews. This close in, you may have slim pickings.

Wow, you decided to go without checking whether any of your points would work or how much hotels would cost???

Club Carlson has no hotels anywhere in Hawaii. So your Club Carlson points are useless.

30k Marriott points is not enough for even one night at any hotels on Kauai, and they don't even seem to have award availability left in case you wanted to use more points (you can only book awards when the "standard" room is available; if the cheapest rooms are sold out and only more expensive rooms are available, that can rule out booking hotel awards).

IHG does not have any hotels on Kauai either, but at any rate 18k would not be enough for a Hawaii hotel if they had one there. (The only hotel they have on another island is 35k points a night, because once again it's a resort.)

And no, AA miles for hotels is generally a very poor value, compared to saving it for a flight.

So basically you saved on the flight, but now you're likely to pay almost as much for the hotels as the flight would have cost had you paid for it. (It's tricky to find decent hotels in Hawaii below about $200 a night even booking well in advance, and btw I don't even know if you can find any hotels with free breakfasts on Kauai, because there basically is no such thing on Maui. So expect to pay for all meals.)

So I don't understand why you went ahead and booked this flight for so close in, despite much advice not to, despite not having done research on hotels, despite not having done research on things to do once you get there.

One thing to keep in mind: While most airlines charge you a fee to cancel and award reservation, many airlines don't charge a fee to change the date of your travel, as long as you keep the origin and destination the same. So if you're finding that hotels are going to put you out of your budget, you could "effectively cancel" the trip for now by taking any award flights you've booked already and moving them for now to any date with the same award availability within the next 10 months, and then once you pick a new date (after you've done more research and after you know what you want in terms of hotels, length of trip, etc) you could move it a real date (that has availability).

OMG I'm now in a state of panic. I didn't realize hotels cost so much in Hawaii

Looks like I can transfer 35k Amex MR points to SPG and book one night at the St Regis Princeville and then purchase (or transfer) 8k Marriott points and book a Marriott resort there? Then I'll only need to worry about 2 nights.

I went to Kauai for 13 days and didn't see/do everything I would have liked to.

As for activities, that depends very much on the person. What do you like to do?

MR points transfer to SPG at 3:1, so you'd need just over 100K MR points to get 35K SPG points. How many MR points do you have?

One thing I could do is cancel my first class reservation on UA and book economy instead. But the problem is that I've already transfered UR points to United, otherwise I could have transferred to Hyatt and booked their resort

Can anyone help?

Frankly, some of us put forth effort in trying to help you upfront, in cautions about planning, timing, and avoiding the predicament you're now in. Miles and points are a wonderful way to save money, enjoy nicer experiences, and see the world but a modicum of pre-planning and research goes a long way toward using them effectively and efficiently.

Did you read this in the post above regarding rescheduling the flights?:

Do not assume availability in high season, especially for holidays like MLK weekend.

You may want to check out a vacation rental. I believe Airbnb codes as a travel purchase for Arrival card. Plus you can save on some of the high food costs by preparing your own in a rental unit.

I think I'm going a ahead and booking one of the hotels on American Airlines hotel booking website. I won't get a great value but I can earn AA later.

It's late to arrange a rental for MLK weekend.

It's late in the game for planning all phases of this trip! Not impossible though.