Make sure you know how many minutes will fit on a tape or CD, and then listen to it all the way thru once or twice before you burn it. I have a couple mix CDs that i'm not quite happy with because i included songs that i don't still think fit.
Without knowing your boyfriend (or probably even if i did know him) it's hard to suggest songs. Rainbow Rocket Ride is unlike anything i listen to, so i don't know any similar songs. Maybe some Modest Mouse is a little similar, you could listen to The Moon and Antarctica and see if you think any of that would fit.
You could also just ask what songs to put on it, if you're not trying to make it a surprise.