I agree with Veronica...
Collect things as you go, take out menu's from places you eat, ticket from the movies those are great as they have the date and name of the movie. Just keep in mind when you guys are out to grab anything that can be glued down as a keepsake.
My Mom went to Poland I told her this and she brought back a McDonalds french fry wrapper (w/ polish writing on it) and I used it as a pocket on my layout to hold her train tickets. She also brough back a used ketchup packet and I stapled it to the page.
Die cuts are great too but to save money on them check out your LSS (local scrapbook store) sometimes they have die cuts machines you can use for free if you purchase the paper from them.
Also journal, journal, journal... It doesn't have to be long or on every page. Mostly journal dates, places and activities. If something funny happened type up a paragraph or 2 on the computer. You can pick a cool font you like, a color of ink to match your LO, etc. If you guys have a favorite song you can print the lyrics.
Have fun with it!