A couple of things about Yuma's surrounding area:
Algodones is a cool little town with a very relaxed attitude. Full of restaurants and pharmacies. I never felt the "stress" that I did when visiting other border cities such as Tijuana, Mexicali, or Ciudad Juarez. The caches there are very easy to get to, and you would have no trouble grabbing all three in an hour. The boundary monuments are all listed under the Benchmarks section of gc.com and are also very easy to visit. Interesting geographic factoid that I noticed the last time I was there: Algodones is the northernmost city in Mexico, and if you go north, south, east, or west from the middle of town, you will cross the border back into the U.S.
The Barry M. Goldwater Range (BMGR) is located southeast of Yuma and has tons of caches. However, you will need a permit to visit the Range, and yes, it is patrolled by various agencies, and they will stop you and ask to see your permits. No problem, though, since all you have to do is visit this website (
BMGR: Visiting the Range) click on the
Download a permit application here link, print it out, and fill it out. Take it to the gate or one of the other offices listed before you want to go on the Range and you will get your permit. Its the easiest thing in the world and an amazing place to play when you are around Yuma. (Just the fact that there are so many caches in the Range should attest to how easy it is to get the permit.)