Congrats!
Picnic? Get MIL to take Flick overnight and go off to nice park with lovely picnic and btl bubbly (or 2!) if the weathers nice, and then home for some fun lol? You could be really cheesy and cut his sandwhiches into hearts
Erm, could you afford a night away in a hotel midweek & have a nice dinner? Groupon sometimes do good deals on midweek hotel breaks.
Do something silly together that you would have done when dating eg bowling, ice skating, horse riding, go karting? Make a day of it eg do some activity in afternoon, go out for meal and then maybe cinema.
I know some of what I suggested is expensive but you could pick a few/mix it up. x