There are several websites that can do this for you. Honeyfund.com, uponourstar.com and traverlersjoy.com are some of the popular ones.
A few things to consider before doing one:
1. They do have transaction fees, so regardless, you will rarely get 100% of the gift. Honeyfund.com is the cheapest as far as fees go because you have the option to link it to your paypal which is only a 3% fee, most of the other sites charge at least 7%. They also give the option for guests to essentially just print out a voucher of what they purchased and write a check/cash directly.
2. Make sure your guests would receive this well. Some people are more open to it than others. Some people are offended by cash or honeymoon registries and consider them an etiquette faux pas, while others think they are a great idea and love to buy activities for the couple.
3. DO NOT count on this money to pay for your actual vacation. The only things that are appropriate to register for are extras like you mentioned. Even then, be sure that you can personally afford to do
some extras on your HM without help. If you get there and can't afford to eat because no one gave towards your registry, then you'd be kind of screwed.
4. Send thank you notes with pictures of you actually doing the activities that people gave towards. This will reassure them that you weren't just trying to deceive them and that you used their gift the way it was intended.
5. Create a small traditional registry for the people who prefer to give a tangible gift. Just because you don't register, doesn't mean that people won't try to find you an actual gift.
6. If you are having only a very small traditional registry, you need to decline all showers. The point of a shower is to 'shower' the bride with gifts. It's really boring to watch the bride open cards...
7. This can be the most expensive way to book a trip. Remember that travel agencies work on commission. You're better off booking directly with a resort.
(credit to Jemini6 & RetreadBride)