Tips: Heads-up —
15 Aug is a public holiday in Poland (Assumption + Armed Forces Day): many shops shut and roads/coast are busier, so leave a little earlier if traffic looks heavy. In Szczecin,
Kasprowicz Park has a big playground and boating lake; the Wały Chrobrego waterfront terraces are a lovely evening walk.
Guarded overnight parking near Budziszyńska 31 (both are open guarded lots, so the roof box is no problem — no height barriers):
1) Parking strzeżony, Zygmunta Chmielewskiego 17 — closest, ~600 m away, 24h, inexpensive (“left a car here a week, no problems”).
Open in Maps ↗ 2) Parking Strzeżony 24h Centrum Kosmos, al. Wojska Polskiego 8 — ~3 km, nearer the centre, 4.5★, takes euros (~1 €/h) and fits tall vehicles.
Open in Maps ↗ Reckon on ~25–40 zł (£5–8) a night; a quick call ahead (esp. the public holiday) confirms a space.