The essentials
A European power adapter, sunscreen rated for genuine midday Mediterranean sun, and comfortable shoes that can handle gravel, cobblestone, or uneven finca terrain, heels included, but pack a backup flat for outdoor moments.
For the wedding day itself
Lightweight formal wear that breathes in summer heat, a light layer or wrap for after sunset, evenings cool down faster than the daytime heat suggests, and a small crossbody or clutch, since many venues are outdoors with limited coat-check-style storage.
For the days around it
Swimwear, most multi-day weddings include at least one pool or beach afternoon, and one nicer daytime outfit for a welcome lunch or dinner that's dressier than beachwear but more relaxed than the wedding itself.