Leave swimming inflatables and toys for children at the pool.
Take new parents a date night kit (for example a film, popcorn, drink and sweets) and babysit so they can enjoy it.
Event organisers/party planners always worry no-one will take part - so show your support and join in, that bake-off needs you!
Go to a local race or marathon and cheer the people running, particularly those that look exhausted and in need of some moral support.
Write to your favourite author, or the author of a book you really enjoyed, and tell them how good their writing is.