I completely understand where you are coming from, as I too struggle with the travel toy scenario where you would like to have something in case you need it, but don't want to bring along stuff that never gets used. First off, I'm not saying that you will even find that you need to have toys along. You may find your days filled with activities and even just the moving from activity to activity (walking to the kid's areas, heading to dinner, etc.) entertains most children and helps to tire them out. That being said, you may find that there are a few minutes of down time in your cabin from time to time where you
may want to have a couple of tricks to pull out. I have definitely had times when travelling when my son just needed some quiet personal time. He didn't need any more screen time or busy time, he just needed time to do something quiet away from all of the hubbub. For those times, I would make the following considerations...
Pack new to them (you may be able to borrow from a friend or purchase second hand), unseen toys for both kids, keeping their interests in mind. You seem to have identified
Lego as one of your 4 year-old's passions. That is the same as my son. On our most recent trip, I purchased a
Lay-n-Go Lite mat for him. It is a small mat that opens up into a decent-sized play area which, when left slightly cinched, provides a shallow rim to keep all pieces contained within the play surface. (Side note: The low reviews on
Amazon "complain" that the mat is too small, but we did not find that at all. One of the reasons why I bought it was because it is small. The playing surface is quite large when open and we found it held a fair number of Lego pieces or other items. It has easily held a 600 piece set of Plus Plus for our 6 year-old in lieu of the Lego.) Anyway, in addition to the Lay-n-Go Lite mat, I also picked up three Lego Mixel sets at $5/piece. These are small sets that are released in a series of colours. Each bag will make one "character," but if you have all three sets from one colour, they can be combined to make one large "max" character, whose building instructions can be found online. The different coloured sets can also be mixed and matched if you would like some colour variety in your pieces. When removed from their main exterior bags, I could easily fit four Mixel sets into the mat and cinch it up. I haven't experimented with how many it would hold, but I would guess up to six (each set has approximately 50 to 70 pieces). On our trip, I gave my son one set each day. He built the characters and also spent a lot of time just building his own creations with the different pieces. We got a lot of mileage out of that one set-up. My son played on airplane tray tables, the floor, and even on the bed. I did find it helpful to take along a few plastic lunch baggies to store pieces from one set in while my son built his next one.
For your youngest, I would try and find a couple of toys that could do double duty. You don't mention if you have a "young" 2 year-old, or one who is heading towards 3. One of our favourite toys at that age was the
Learning Resources Smart Snacks Trail Mix & Match set. The pieces are great for lacing for a slightly older child, they can act as a play food, or even provide entertainment for younger kids who love to "fill and spill" a bag of items. The great thing is that it comes with its own bag to contain all of the pieces and doesn't take up much space. If you think a simpler lacing set would work better, you could try the
ALEX Toys Little Hands String A Farm (or other themes, such as String and Beep for vehicles). These pieces are chunkier and can act as toys themselves, or be stacked by an adult for knocking down. Other suggestions include a small set of stacking cups like the Munchkin Caterpillar Spinners that I found for a few dollars at Target (can be used as tub toys, for stacking, or for hiding a toy underneath and then finding which one the surprise is under), a set of DK Touch and Feel cards (these are great...we liked the First Words and Animal ones) or a
Disney First Look and Find book (these are a larger, hardcover/board book, but my 2 year-old loved the ones we had and would spend a lot of time looking at them with me or also alone.) We also had good success with a little
Hape wooden cutting food set that I picked up. My son loved slicing them apart and pairing them back together like a puzzle.
Anyway, those are just a few of my small space restrictions/high interest suggestions that worked for us, should you decide to bring entertainment along. I would second the recommendations for bringing along a favourite stuffed animal as well if those are important to your kids, and I would only bring along colouring books, crayons, or sticker books if those kinds of activities truly impress your kids, otherwise, they are a waste of space. Only you know your children and what may hold their attention long enough for it to be worth your time to bring on vacation with you.