Depends on what you're hoping to get from it. You still won't be married or have the legal protection that gives you.
If that doesn't matter and you've taken the other necessary steps to protect yourselves, as above, then just do whatever feels right.
You could have a humanist ceremony. Order the book from the humanist society website which has a range of different types of services as a useful starting point. You could have a service which includes the DC if that would be meaningful. You could have music or readings, have a handfasting or light a symbolic candle, say specific meaningful words to each other, exchange rings or other significant tokens, sign a certificate to commemorate the day.
A humanist wedding isn't legal in England, though it is in Scotland, but I've been to two "weddings" with a humanist celebrant which you couldn't tell weren't official, where the couple did the legal bit in a 10 minute registry office thing beforehand.
Are you ideologically opposed to legal marriage? If not, you can make your wedding as meaningful, personal or off beat as you like. If you are and the state stamped bit of it doesn't sit right, I'd sort wills, deeds, insurance etc if you haven't, then have a service of sorts or a special ceremony combining none, some or all of the elements above to celebrate your love and commitment and invite people or just be you guys as appropriate.