Day trips are short 2-5 hour trips during single day. On these trips its most important to dress up according weather and with enough layers. It is colder on the lakes than at the land. During sunny days its good to realize that burning in the sun is easy at the lake.
For day trip pack at least:

Extra clothes/warm clothes
Sailing/gardening/work gloves
Water bottle and possible small snacks
Sunglasses and hat on sunny days

You can borrow flotation vest at the boats, but you can also bring your own life vest with you.

Overnight sailing trips

During overnight sailing trips people usually sleep inside the boat. Sometimes a tent might be needed if there are a lot of people, but this is usually avoided and will be informed specially. Boats have mattresses, but no blankets, pillows or bed sheets. Food is cooked at the boat or at fire at the islands. The skipper of the boat organises the provisioning (=getting the food).

Packing should be done carefully. Everyone should have everything you need, but packing extra stuff reduces the already little space that boats have.

You should pack at least:

Enough weather appropriate clothing:
– Enough change of clothes
– Warm under layer
– Clothes for rain
– Hat of some sort
– Sailing or gardening/work gloves
Swimsuit and towel for sauna and swimming
Sleeping bag or blanket and bedsheets
Flashlight or headlamp at least during autumn
Personal medication
Hygiene stuff
Water bottle
USB-A cable* for charging the phone
Some cash (Peronsaari for example only accept cash)
*USB-A cable suitable for boat ports

You can also pack:

Pillow (sails and life vests are good pillow replacements and don’t take any extra space)
Own alcoholic/non-alcoholic drinks
Mosquito repellant