are that I'll think on this over the weekend!
First thoughts were for a surprise party (did one for my Other Half for his 50th - great time had by all) but it does take one heck of a lot of organising and planning. After minitot has arrived, you may not feel up to it right then. Way to get round? Surprise party, with all the invitees bringing home cooked food and drink on a voluntary basis. If they're worth inviting, they should be offering to do this anyhow, they will all know about your recent arrival!
No matter what, seriously think about involving the kids in "doing the house up" - i.e. decorating as much of the outside and inside of the house as you can. You can also get them to do home made decorations/posters etc. Loadsa balloons all over the place, and what really gives a red face is the largest banner you can find with "XXXX IS 40 TODAY" etc. (Had 50th ones plastered over cars, front of house, windows, balloons tied to gate, doorhandles etc. Chimney wasn't practical - too high!)
Another alternative is to turn it into a "disco", as he's into music, or even theme it as a 70's or 80's night. If you let him take over spinning the discs, he can play all his favourites, too!
As a pressie from you, Jessie, how about cutting out 40 hearts - each one can hold an IOU, a promise etc. (I'm hopeless at explaining what I'm thinking, sorry!) An IOU can be for washing the car, mowing the lawn etc. A promise for a sleep-in, pay for a pool game etc. These things don't cost a lot, but say much. Obviously, you can add more sentimental things i.e. IOU a love poem etc. Jessie, you don't need ME to tell you what to put into the soppy ones!!
Still thinking on the quiet nite theme too. In that case, hide all 40 heart messages, and create a treasure hunt! I agree with JD, you may find that's the only quiet, quality time you've had together since the arrival of minitot.
Will see what else my befuddled brain can come up with over the weekend. Hope this gives you some inspiration, Jessie!