D.T.
By 18 months all my kids were pretty much self-entertaining. They are BUSY at that age. Very active. First thing to do is kid-proof 1 or 2 rooms in your house and keep him in there. If you don't, you will probably find yourself saying "don't" - repeatedly - all afternoon. Not fun for you or him. If they hear it too much, they tune it out... so save the "no's" and "don'ts" to just a couple times - for the important stuff (like when they try to run into the road).
As for toys.... empty boxes, wooden spoons, mixing bowls, plastic measuring cups, and other misc stuff from your kitchen will probably keep him entertained for quite some time. No need to buy anything special for him if it's just a one-time thing. Simple open-ended toys are best for all ages, but especially toddlers: wooden blocks, small and large balls, play-doh, cars, trucks, Little People stuff, etc. In general it's best to avoid stuff that requires batteries as they are close-ended toys, meaning kids only learn "I press this button and that happens"... and they don't keep the kids' attention span very long. Another good idea is to set up an obstacle course of sorts... pile pillows on the floor and let him crawl/walk over them. Let him try to kick a ball... don't worry... an 18 month old can't kick hard enough to break anything (assuming you kid-proofed decently). Let him try to teach the baby to crawl... toddlers love being the 'big kid' and teaching babies things and the baby will probably be fascinated watching him run and be inspired to try to crawl/stand.
I would avoid DVDs and tv for this age if at all possible. But, as the mother of young children, I do understand how sometimes it's nice to put one in to keep the older one in place while attending to the baby. Imagination Movers, Word World, SuperWhy and Wonder Pets were the favorites of my kids at that age (and still are favorites at 2 and 4). Oh... my 7 year old loved Bob the Builder when he was that age, too.