This looks quite cool!
I think it would be most useful in later stages of planning and sharing the result with friends. The major steps of planning a big trip (as I see it) are:
1. figuring out what activity options there are (brainstorming+researching what to do)
2. getting that into a sensible schedule with all constraints of opening hours, distance/travel between them, available time in the trip, budgets (either the glacier tour or that lodge with a skylight to see auroras, say), and preferences (some activities are cooler than two other ideas together)
3. booking
4. doing, and adapting on the fly as plans meet reality
I did these things in a Markdown file last time (CodiMD, specifically) and that worked surprisingly well. The big downside is that you can't have different views on the same data: to get from step 1 to 2, it would be cool if you can plot them on a map as an approximation of how much time you need to travel between them, or to give them all coordinates and have it show you a traveling salesman solution, just to have a starting point of what order makes sense. Or URLs for everything (info, reviews, booking status, ticket download..) gets messy. Ikuyo can view activities as a list and also as a map and also as a schedule, so that looks very promising!
Making a test trip, besides some unnecessary metadata like currencies (same, all euros) and target region ("various countries in Schengen" was not an option), I didn't really feel like it helped me get any of the stages done specifically. You need to pick a datetime for each activity up front, or you leave it as dummy, but either way you can't then drag&drop events to make a schedule. You need to open them up, press edit, and use the time selector UI. In Markdown, moving an event isn't drag&drop either but at least I can simply move the line to where on the schedule it makes sense and type the time directly
So I guess I'd put the activities in here once I've got a draft in Markdown? Then friends can view it in various ways. A friend previously (different trip than where I used Md) used a spreadsheet and that experience was relatively mediocre: Ikuyo would most definitely be better as a sharing format!
For during the trip, is there a way to save it offline? Comparing to Markdown or spreadsheets, I can download an HTML render or ODS file on my phone and view it any time, or even print it out. For viewing my trip on this website randomly during the trip ("what time did we need to be back in the village again?"), you'll always need internet and the website to be up right?
I had a peek at the source code as well and it looks all neat. After logging in, I needed to edit the URL to get back to the homepage (clicking the logo doesn't do it) and find the source link, but that's fine for me. The number of dependencies looks pretty reasonable and I could see myself hosting this!