Croatia in May: Zagreb, Rijeka, Krk

Hello! I’m planning a solo long weekend trip for 23-25 May and would love some advice.

I’ll fly from Sofia to Zagreb, arriving Saturday around 10:00. Plan is to explore Zagreb a bit, have lunch, then take a bus/train to Rijeka staying there for 2 nights.

I’m mainly after coastal vibes: beach promenades, sea air, relaxing walks. Sofia is landlocked and I’m a bit bored of cities/mountains, so the Adriatic is my main draw. Zagreb kind if reminds me of Bratislava where I lived for a bit and maybe it would be interesting?

Initially I planned just Rijeka, but Krk island is nearby and more interesting for beaches. I’ve never been to an island before, so I’m considering a day trip. I don’t care about swimming/sunbathing,just walking along the shore.

I have a few questions:

1. Is late May suitable for walking on beaches with regular shoes, or are pebble sandals needed? Also, would it be odd/unacceptable to walk with shoes if there are sunbathers?

2. Would you recommend Krk or somewhere like Opatija for better coastal scenery like beaches and hills?

3. I’ll rely on public transport. Are buses the best option between Zagreb, Rijeka, and Krk?

My rough plan is like this:

Saturday: arrive Zagreb -> Rijeka

Sunday: Rijeka + possibly Krk

Monday: more Rijeka then Zagreb or straight back to Zagreb (flight at 22:30)

Does this sound doable for ~3 days?

I’m skipping car rental as I don’t like driving unfamiliar roads. Would it make more sense to skip Zagreb entirely and focus on the coast?

Also, is it risky/expensive to book accommodation and transport last-minute in late May, or is it fine to stay flexible?

Thanks!