things.py

from django.db import models
from modelcluster.fields import ParentalKey
from wagtail.wagtailcore.models import Page, Orderable
from wagtail.wagtailadmin.edit_handlers import FieldPanel, InlinePanel


class Thing(models.Model):
    ...


class PageThings(models.Model): # Inherit from Orderable if you would like reorder
    page = ParentalKey('myapp.MyPage', related_name='things')
    thing = models.ForeignKey(Thing, related_name='pages')

    panels = [
        FieldPanel('thing')
    ]

class MyPage(Page):
    ...

    def get_things(self):
        return Thing.objects.filter(pages__page=self)

MyPage.content_panels = [
    InlinePanel(MyPage, 'things', label="Things"),
    ...
]